Saturday, March 12, 2016

Blaster modifications and safety in the context of HvZ

This document is intended as a reference for anyone who needs to decide what modifications to Nerf blasters (and similar toys from other brands) to permit in their game, with a focus on games of HvZ played on campuses. The purpose of this guide is to give people the knowledge that they need to make safe decisions without being overly restrictive. It was originally supposed to be short and concise, but it grew over time into, well, this. Safety is important, so I think it's worth taking the time to read. This may be of use to both moderators, who make the details of and implement a game's rules, and campus officials, who have the final say on what is permissible.

I've added pictures in an attempt to make this a little less wall-of-text-ish and to break up the sections; these images are from Steph Smith's pictures from the University of Waterloo's first invitational, this imgur album, Oklahoma State University HvZ Facebook page, more from the same facebook page, and All of the images you see here have been edited (trimmed and resized) by myself.

EDIT: Almost immediately after publishing this  post, a conversation started on reddit regarding the use and safety of Chinese LiCo batteries such as Trustfires - which has been a matter of some controversy over the years in the Nerf and other hobby electronics communities. I may have been overly generous in my assessment of the safety of these batteries, so I've edited the post to correct this.



Modifications: Velocity increase

When people talk about modified blasters, they usually mean blasters modified to launch darts (or disks, balls, etc.) at a higher velocity, which increases their effective range, makes them harder to dodge, and makes the impacts easier to notice. There are as many ways of doing this as there are ways to fling foam - that is to say, a lot - but the end result is always the same: the blaster launches projectiles with a higher (in some cases, much higher) velocity. Some modifications that players will wish to do for other reasons have the side effect of slightly increasing velocity (see Misc. notes: flywheel blasters).

Stock Nerf blasters shoot at a low velocity because children cannot be trusted not to shoot each other in the eyes at point-blank range. As HvZ is played by young adults, blasters modified for velocities higher than stock can be and often are safely used. Of course, there should be an upper limit of some sort on how hard a blaster may shoot. In a typical game of HvZ, which is played in a public space, this upper limit exists mainly to protect bystanders. Player comfort is also a concern as HvZ attracts casual players, but most players will be full of adrenaline and will not mind (and may not even notice!) a brief sting from a high-velocity dart. A much higher velocity threshold is suitable for (rare for HvZ) situations where avoiding actual injury is the only concern.

There are two ways that this limit can be implemented:
  • Modified blasters may be fired over a chronometer. This has the advantage of being objective, and the disadvantage of requiring a chronometer. Overall, this system is preferable.
  • One or more moderators can be shot with a modified blaster; if they say "ouch", the blaster cannot be used. This has the advantage of being easy to implement and the disadvantage of being subjective.
These systems can be combined. For example, you might use a chronograph as your first option, and require a moderator pain test for blasters which fire new or unusual projectiles.

Hasbro's half-inch-diameter foam darts are by far the most common type of projectile that players will want to use, along with Chinese imitations thereof. If you use a chronograph, 130 fps with such darts is a good limit for most games, as it is easy for players to build blasters which perform close to, but not above, this limit (see Misc. notes: flywheel blasters). The velocity produced by a blaster can vary from shot to shot; a variation over a range of up to a few dozen fps is normal. Blasters that initially test close to the limit should be tested multiple times, because if such blasters are only tested with one shot, this introduces a significant element of randomness into the testing procedure. Please either be lenient with blasters that occasionally shoot a few fps above the limit so long as they usually shoot below it and never shoot far above it, or increase the limit to 135 or 140 fps to accommodate this randomness.

Objectivity and subjectivity deserve further elaboration. Subjectivity in the rules and regulations is problematic because it creates uncertainty - and this uncertainty cuts both ways. Players cannot be sure that their blasters will not be unexpectedly banned if there are not well-defined and measurable criteria for determining permissibly, and there is a risk that an especially pain-tolerant moderator may allow a blaster which should have been banned. The best way to mitigate this problem is to eliminate subjectivity wherever possible - hence the use of a chronograph, with multiple shots tested for blasters that are close to the limit, being preferable. If a pain test is used, subjectivity can be reduced by having multiple moderators involved. For example, the rules for HvZ at Mount Allison require three moderators to approve each modified blaster.

Other velocity limits are appropriate for other projectiles. Such velocity limits should be informed by a pain test conducted by the moderators. Beware that some zombie players will complain that projectiles hurt when they actually want to reduce the effectiveness of blasters in human hands to make them game easier for them. Kinetic energy density - defined as the kinetic energy per surface area of the tip of the projectile when compressed as it would be on impact - is only a crude measure of pain on impact, as the softness of a tip can make a large difference. It should be noted that sufficiently massive projectiles - in particular, Zing arrows - can break some windows at velocities which do not cause pain. The UK-based group Green Cloaks uses a velocity limit of 130 fps for .5 caliber foam darts, 95 fps for .7 caliber foam darts, and 65 fps for foam "missiles" (very large darts, typically with fins).

Some blasters that have been modified for increased velocity have brass (or other metal) barrels, which can extend beyond the front of the blaster. If these barrels are not sheathed (plastic piping works well for this), there is a risk that a player may be cut by the end of the barrel if they forcefully collide with the blaster's user. Unsheathed protruding metal barrels should not be allowed.

Modifications: Rate of fire increase

Most Nerf blasters are either pump action, manual single-action, or semi automatic. Therefore, their rate of fire is entirely dependent on the player who is using them. However, there are some fully automatic blasters, and the rate of fire of these blasters can be increased independently of their velocity.

A blaster which has been modified for increased rate of fire may look like one which has been modified for increased velocity. So, if you have banned modification for increased velocity and see a player with e.g. battery packs on the sides of their blaster, don't panic! Their blaster may simply have been modified for an increased rate of fire.

Modifications: Reliability improvement

Most blasters contain locks which are intended to prevent users from operating the blaster in ways that could damage the blaster or let them pinch their own fingers. These are potential points of failure that make the blaster less reliable, and they often create friction which makes a blaster function less smoothly. In addition, some blasters are not well-designed or have manufacturing problems which modification can fix.

Reliability improvements, including lock removals, are safe. The worst that could possibly result is that a very careless user might pinch their own fingers, which hurts but does not result in lasting injury. It is often impossible to tell whether a blaster has had a lock removed simply by looking at it. A rule which bans reliability improvement modifications is not only pointless, but also unenforceable.

Electric Nerf blasters come with low-quality metal motor brushes. Replacing these with carbon brushes allows the motors to last longer, and is especially helpful if a higher voltage is used. A brush replacement by itself will, for all practical purposes, only increase reliability: it will slightly reduce the friction on the motor shaft, but the performance gains from this are negligible.

Modifications: Darts (aftermarket, modified, and homemade)

The darts that Hasbro makes are not aerodynamically stable at high velocities, and cannot be bought cheaply in bulk. Players may wish to modify their darts to make them more stable at high velocities, or use aftermarket or homemade darts to either increase stability or save money. Aftermarket, modified, and homemade darts exist in a full spectrum from darts which are just as safe and pain-free as Hasbro's darts, to darts which are safe in that there is no risk of serious injury but that can hurt or cause minor bruising on impact, to darts which should not be used without eye protection.

Here are several types of aftermarket, modified, and homemade darts which your players might wish to use:
  • Koosh darts are so called because their tips resemble the surface of a koosh ball. They are accurate and hurt no more than Hasbro's darts. I recommend allowing these darts in all games.
  • Vobbery darts resemble Hasbro's darts, except that the foam comes in different colours and they are heavier, which makes them more prone to hurt on impact. However, players may wish to use them because they are very cheap.
  • Full vinyl jacket darts are so called because they have hard tips. They otherwise resemble Hasbro's darts. They hurt on impact and I recommend banning them unless your players are pain-tolerant.
  • Modified Hasbro darts are a wildcard. They might be completely safe, they might hurt more on impact, or they might have heads which fall off to reveal sharp metal weights. You aren't very likely to see these darts nowadays, given the availability of cheap Chinese aftermarket darts. I recommend that you ban them.
  • Stefans are homemade darts that are typically used in high-powered homemade blasters. They are typically made from foam backer rod, weighted with either a washer or a BB, and have either a felt coating or hotglue dome on the tip. They hurt on impact and you do not want them in your game unless your players are wearing eye protection and are very pain-tolerant.
Modifications to other types of ammo (disks, missiles, etc.) are rare.

If you want to reduce the potential for pain on impact of your player's darts, please ban heavy and hard-tipped darts instead of reducing the allowable muzzle velocity. The Green Cloaks bans all projectiles with a hard tip, or that have been modified by the player, and imposes a limits of 1.25g, 2.5g, and 8.5g for .5 cal darts, .7 cal darts, and missiles respectively. Projectiles that have a toy CE rating are exempt from this mass limit. Under these rules, Hasbro's projectiles and koosh darts are permitted, and vobbery darts are not.

Modifications: Shell modifications

There are several commonly-performed modifications which alter the shell of a blaster and which do not affect the way that the blaster fires:
  • Integration, meaning the permanent combination of several blasters (or a flashlight, etc.) into a single unit. This is completely safe.
  • Ergonomic improvement. This is safe, assuming that no sharp edges are exposed.
  • Painting or otherwise decorating blasters. This is safe so long as safe and toy-like colours are used, but it is possible to paint a blaster in such a way that it could be mistaken for a weapon. You might choose to trust moderators to judge each blaster individually, or you may want to make more specific rules. For example, you might require >50% of the visible surface to be white, a bright high-saturation colour, or transparent (though it is noteworthy that some stock Nerf blasters would not meet this rule!).
  • Jam door removal and jam door hole enlargement, which expedites clearing jams and allows magazines to be reloaded while still in the blaster. Jam door removal makes it possible for a very careless player to pinch their own fingers; it is otherwise safe. It also requires that the jam door lock be removed.

Misc. notes


Misc. notes: Flywheel blasters

Electronic flywheel blaster deserve special mention for two reasons. First, there is an upper limit on how hard flywheel dart blasters with a single pair of flywheels can shoot, called the glass ceiling or critical velocity, of about 110 to 120 fps (or 130 fps with certain darts). Beyond this point, increasing the speed of the flywheels further has little effect on the speed of the dart as the dart simply slips in the flywheels. In theory, this is a hard upper limit because the coefficient of dynamic friction does not vary with slippage speed. In practice, well . . . it's complicated, but it suffices to say that the velocity gains to be had from a massively supercritical flywheel surface speed are small if they exist at all.

The glass ceiling is an inherent property of the flywheel geometry and dart construction. Nothing short of altering the cage or flywheels to reduce the gap or increase the contact area, or using different darts, can raise or lower the glass ceiling. The glass ceiling is generally independent of flywheel surface material, as flywheels develop a layer of melted foam from darts as they are used, though flywheel surface material may be significant if it prevents this buildup (e.g. as it does with aftermarket aluminum flywheels).

It is therefore convenient to have a maximum dart velocity that is just a little above this glass ceiling, as this both makes evaluating blasters easy for moderators (Single-stage flywheel? No modifications to flywheel or cage geometry? No need to test, good to go!) and makes it easy for players to build blasters which will consistently perform close to, but not above, this limit.

Furthermore, due to the speed-vs-torque curves of DC electric motors, supercritical blasters have better windup response and ability to resist velocity droop during rapid fire. Players will want to use blasters that brush against this glass ceiling. In short, a velocity limit with darts that is a bit above the glass ceiling makes both players and moderators happy.

Second, rewiring a flywheel blaster to improve reliability, with no other modifications, will typically slightly increase velocity because doing so removes sources of parasitic resistance. Players may also wish to rewire their blasters so that they can accept higher-voltage power sources, even if they do not use a higher than stock voltage in your game. Likewise, the use of battery packs will tend to increase velocity (because matching the velocity achievable with alkaline cells exactly would be difficult, and players are unlikely to be willing to put up with a decreased velocity). Battery packs are used in modified blasters because they are a more efficient, compact, and cheap (in the long run) power source than individual cells in trays with spring contacts.

So, if you decide to ban any and all velocity increase modifications, you will cause users of flywheel blasters a lot of grief, because some of the modifications that they would like to perform for other reasons would have the side effect of increasing velocity.

Misc. notes: Pneumatic blasters

Pneumatic blasters come in two varieties: manual single-action and automatic. Manual single-action blasters must be pumped several times before firing, and, depending on how many times they are pumped before firing, can fire at differing velocities. Beware of this, and be sure to pump manual single-action pneumatic blasters fully when testing them. Be suspicious if anyone claims that they will only pump their blaster a specific, limited, number of times during a game: it is very easy for a player to forget (or "forget") how many times they have pumped their blaster!

With automatic pneumatic blasters, feeding the firing mechanism air at a higher pressure results in the blaster firing more rapidly, but not at a significantly higher velocity. The velocity of such blasters can be increased by modifying the firing mechanism, but modifying the air supply can only significantly affect ROF and endurance.

Replacing a blaster's pump does not affect its safety; although a new pump may be theoretically capable of producing higher pressures, an overpressure release valve (OPRV) should set the upper limit on the pressure that the system may be required to withstand. An electric pump is as safe as its power supply (see Electrical safety). It ultimately does not matter where air pressure comes from so long is it is stored and used safely.

Concerns have been expressed about the possibility of a player attempting to modify a pneumatic blaster and accidentally making a plastic pipe bomb. In practice, this is not a problem - modders who work with pneumatics generally know what they are doing, and those that don't make systems that leak and thus don't work at all. However, as this is a dire possibility, precautions are warranted despite the very low probability of disaster:
  • All air systems, whether homemade or modified, must have some sort of overpressure protection. Leaving in stock OPRVs in toys and the use of appropriately rated overpressure protection systems are both acceptable. 
  • All fittings, valves, tanks, etc. used in homemade air systems must be rated for the highest gas pressure that they might be required to withstand. 
  • Air tanks made from PVC piping are dangerous. PVC pipes can be safely used with pressurized water because water is incomprehensible and thus does not store energy when under pressure - but air does store energy, and this energy can create and propel shrapnel during a sudden failure of a PVC part. PVC creates sharp edges when it breaks under pressure and, just to add insult to injury, PVC does not show up well on medical X-rays. Seriously, ban this stuff, at least as far a pressure systems are concerned. (PVC barrels, plunger tubes, etc. are fine.)

Misc. notes: The Rival line

The Rival line is a line of blasters released by Hasbro which are marketed to children aged 14+. Rival blasters use solid foam spherical ammunition called rounds. As of the time of this writing, only two Rival blasters have been released - one spring powered (the Apollo), and one flywheel powered (the Zeus). Both claim to shoot 100 FPS out of the box, but in fact shoot closer to 90 FPS. Another two have been announced, one which also uses flywheels (the Khaos), and another springer (the Atlas).

The Apollo is difficult to modify for increased velocity, and the glass ceiling velocity for the Zeus is about 115 FPS - not that much higher than the 90 FPS that it shoots out of the box!

Rounds are heavier than darts, but have a larger and softer surface of impact due to being compressible spheres. Subjectively, rounds feel roughly the same on impact at point-blank range as the Hasbo Elite dart - the most commonly used dart type - does at a typical glass ceiling velocity.

I recommend that you allow both stock and modified Rival blasters in your game if you allow blasters that have been modified for higher velocity.

Electrical safety

Electrical power system safety: What could go wrong?

Certain high-performance batteries - specifically, lithium ion - may undergo an incendiary failure if mistreated, leading to property damage. (It is safe to assume that a player will drop their blaster if it starts spewing flames!) An incendiary failure resembles a combination of a blowtorch and a Roman candle firework. The main danger of an incendiary failure comes from the possibility of igniting nearby flammables. This entire section covers electrical power system safety, with a focus on the safe use of such batteries, but this is not because the risk of fire associated with modified blasters is high compared to e.g. RC vehicles (it isn't). This is simply because there is a lot of technical information to cover.

The fire risk associated with modified blasters is generally very low, as blasters are not made from dangerously flammable material. A blaster's shell is pretty good at depriving small fires oxygen, and is usually made from ABS plastic, which is hard to ignite and tends to melt away from heat sources instead of burning. The internal parts are made from various plastics, with some metal. Some of the materials found inside blasters (e.g. PVC, a common wire insulator) are effective flame retardants.

Flammable materials around blasters are a greater concern. For example, the same burning battery could lead to nothing of significance beyond a ruined blaster on a paved road, damage and ugly scorch marks on a modern linoleum (which is actually PVC) floor, or a catastrophe in a forest. For this reason, the appropriate safety standards depend on the environment in which a game takes place.

The risk of fire on a typical campus during play is relatively low, as play takes place on pavement, on grass, near flowerbeds, and inside brick and concrete buildings with linoleum tiled floors. Concentrations of flammable materials on campus are typically found in offices, libraries, and dormitories - and play should generally not occur in these places for other reasons.

However, for players who live on campus and store and charge their lithium ion batteries in their dormitories, the risk of fire is greater. RC hobbyist wisdom has it that the majority of incendiary failures occur during charging. Your campus may already have policies regarding the storage and charging of lithium ion batteries in dormitories because they see use in RC vehicles; if so, please ensure that players are aware of these policies.

This guide covers only those aspects of safe lithium ion use that ensure safety during play, as these are under your control as a game organizer. If you are running a Nerf club or such, please ensure that people who use lithium batteries are educated about safe lithium charging, storage, and disposal (which is something that I won't cover here because this guide is already pretty dang long).

Electrical power system safety: Battery types

The battery is by far the most important component of a modified blaster from a fire-safety perspective. While there are a wide variety of battery chemistries which players may wish to use, for our purposes they can be grouped into two categories: lithium ion, and everything else.

Let's get the "everything else" out of the way:
  • Alkaline consumer cells are, for most people, the default option. They are what Nerf blasters are designed to use.
  • Nickel-metal hydride (NiMH): Rechargeable consumer cells with this chemistry can often be bought alongside alkalines, and can be used in a blaster without modification. NiMH battery packs, consisting of hard-cased cells welded together and shrinkwrapped, also exist and are suitable for use in Nerf blasters.
  • Nickel-cadmium (NiCD) cells are a deprecated technology, so you aren't likely to see them. From an environmental standpoint, they are bad if improperly disposed or if they leak. From a fire-safety standpoint, they are equivalent to NiMH.
These batteries are completely safe and should always be allowed. They cannot catch fire no matter how they are abused. In the very worst case, the cells may leak a mildly caustic electrolyte fluid, which is an irritant.

Now, on to lithium ion: these batteries can be useful in modified blasters because they have a high voltage per cell (around 4V when freshly charged), a high energy density, and can have a very high power density. From a safety standpoint, the most notable feature of lithium ion batteries is that an incendiary failure, known as a thermal runaway, is possible if they are severely overheated or damaged. This thermal runaway is a multi-stage process, the last and most destructive of which is the chemical breakdown of the cathode. This last stage does not always occur, but a partial runaway can still be destructive. A thermal runaway is not primarily a combustion reaction (though it can include one) and is not dependent on external oxygen. The key to using lithium ion batteries safely is to ensure that they are never overheated or damaged. 

There are many variations of lithium ion chemistry, some of which are much safer than others. With a relatively safe lithium chemistry, a thermal runaway will be harder to start via overheating (because less waste heat is generated during discharge), a complete thermal runaway will be less likely to occur even if a thermal runaway is initiated (because the cathode chemically breaks down at a higher temperature), and a thermal runaway will be less destructive even in the worst case (because the cathode releases less heat when it chemically breaks down). These characteristics are correlated, which means that lithium chemistries can be broadly categorized as more or less safe with little regard for the technical details.

Here are the variants of lithium ion chemistry that you are likely to encounter, in order from most to least safe:
  • Lithium-iron-phosphate (LiFePO4, also known as IFR or LSP, often abbreviated as lithium-phosphate or LiFe) is a unusual in several ways. It has a lower maximum charge voltage than is usual for lithium - 3.6 V rather than 4.2 V - and is generally much more resilient than other lithium chemistries. No lithium chemistry is completely foolproof, but LiFe comes close. This chemistry is often seen in battery packs intended for RC hobbyists who wish to minimize their risk of a thermal runaway.
  • Lithium-manganese-oxide (LiMn2O4, also known as IMR or LMO, often abbreviated as LiMn or lithium manganese) is often used in power tools and industrial applications. Loose cells with this chemistry are available from industrial suppliers, and lithium ion cells intended for use in flashlights and vaporizers sometimes use this chemistry.
  • Hybrid chemistry is a generic term for lithium ion variants which combine (most of) the high discharge rate of LiMn with (most of) the high capacity of LiCo. 
  • Lithium manganese nickel (LiNiMnCoO2, also known as INR or NMC) is an example of a hybrid chemistry. Variants with differing amounts of manganese, nickel, and cobalt are under development. You aren't likely to see them in modified blasters now, but you might in the years to come.
  • Lithium-cobalt-oxide (LiCoO2, also known as ICR or LCO, often abbreviated as lithium-cobalt or LiCo) is the most commonly used lithium ion variant in consumer electronics because it has the highest energy density. However, it is also the most dangerous variant of lithium ion. You probably have a LiCo battery in your pocket right now, in your cell phone - which has dedicated protective circuitry built in. The LiCo batteries that are sometimes used in modified Nerf blasters usually do not have any protective circuitry.
  • Lithium polymer deserves special mention. All of the other variants of lithium ion on this list are distinguished by their cathode material, and have similar (generally carbon) anodes and similar (organic solvent) electrolytes. "Lithium polymer" can refer to any lithium ion chemistry that uses a special polymer electrolyte, that uses a hybrid between this and a more conventional electrolyte, or which is merely packaged in a polymer case. So: every LiPo is also one of the other chemistries on this list, depending on the cathode material. This is usually LiCo.
If someone shows up with cells that they can only call "lithium ion" and cannot be more specific, it would be prudent to assume the worst.

All of these variants can be used safely with the appropriate precautions (which are covered in the next section). In areas where there no significant risk of fire, a laissez faire approach is acceptable. If there is a risk of fire, the rules outlined in this section should be followed rigorously if lithium ion batteries are used. In areas where the fire risk is very high, it would be wise to outright ban all lithium ion variants except for LiFe and perhaps LiMn.


Electrical power system safety: Lithium, (hopefully not) overloaded


The discharge rating of a battery is a measure of the amount of current that a battery can produce without significant voltage droop and, more importantly for our purposes, how much current it can safely sustain. Using a lithium ion battery with an inadequate discharge rating can cause overheating and is dangerous. (A high discharge rating is never a problem. Overloading is purely a performance and not a safety concern for other chemistries.)

The current which may be drawn from a battery depends on the blaster in which it is used. Manually verifying the discharge rating of every lithium ion battery and the current which may be drawn by every blaster would be an enormous hassle. Fortunately, this is not necessary, as there are some quick and easy to apply rules which cover the majority of cases:
  • OK: Any battery that isn't lithium ion
  • OK: Battery packs (but check the discharge rating if a lithium ion pack is visibly concerningly tiny)
  • OK: AA sized LiMn (commonly known as "IMR 14500") cells in flywheel dart blasters with stock motors or other low current applications.
  • Probably OK, but only probably: AA sized LiCo cells used in very low current applications such as powering LEDs or relays. Loose lithium ion cells may be assumed to be LiCo if the chemistry is not specified, especially if the are of the Ultrafire and Trustfire brands.
  • Probably OK, but only probably: Protected AA sized LiCo cells, meaning cells with protective circuitry built-in. Any cells which do not say "protected" or "with PCB" on the wrapper may be assumed to be unprotected.
  • Bad: AA LiMn or LiCo that don't fall in to one of the above categories.
These are rules that can be applied quickly, and may be inapplicable in a small subset of cases. If a player is using some type of AA lithium ion cell other than the above, or a pack that is looks concerningly tiny, or thinks that their blaster is safe despite not following the above guidelines, then it is necessary to asses the blaster in more depth.

The exact current that may be drawn by a blaster is difficult to calculate exactly, and in some cases is difficult to measure even if you have the blaster and an ammeter in front of you because it varies quickly over time. As a rough but good enough rule of thumb, electric spring-powered blasters require about 10 A and flywheel blasters with stock motors require about 5 A. As a conservative rule of thumb, flywheel blasters with upgraded motors can be treated as if they draw current equal to their motors' rated current at maximum efficiency, doubled (because there are two motors; one for each flywheel). Blasters draw a brief spike of very high current when beginning to fire or revving up; since this spike is too brief to cause significant heating, it is OK if it exceeds a battery's rated discharge rate.

All of the lithium ion batteries which you are likely to see come in two configurations, which can be handled separately: loose AA cells, and packs.

Loose AA cells are favored by novice modders, as blasters that are designed to be used with AA consumer cells can use them with little or no modification. Conveniently, the all of the AA sized cells that you are likely to see fall into two categories: Trustfires and IMR 14500 cells.

Trustfire is a generic term in the Nerf modding community for AA sized LiCo cells of the Trustfire and Ultrafire brands, which are made in China. These cells have a low current rating, or have no stated rating at all, and have a reputation for overstating their capacity on the label. They are also of dubious manufacturing quality. These cells see use in Nerf blasters because they are cheap, are easy to install, and just "work well enough." They haven't burnt any houses down - yet - but why play with fire?

Trustfires come in two varieties: protected cells which have a protection circuit to prevent overdischarge and overloading built in, and unprotected cells which do not. It should be noted that some cells which claim to be protected actually have no protection circuit - this is possible because of loose regulations in China. Due to these same loose regulations, the efficacy of the protective circuits on cells that do have them is questionable. Even if a player could be sure that their cells come from a good manufacturer, it would be impossible to verify this as a moderator due to the possibility of look-alike fakes. Protected (and "protected") cells are generally not used because they do not always fit into standard AA trays and the protection circuit can trip during normal use of a blaster.

If you allow LiCo cells in you game at all, they should only be used in low-current applications such as powering LEDs or relays. The UK-based group Green Cloaks used to allow protected Trustfires, but have changed their rules to outright ban LiCo.

IMR 14500 cells are better. IMR, as you may recall, specifies that the chemistry is lithium manganese, which is often used in power tools. 14500 is an industrial cell size which is the same as consumer AAs. These batteries are typically rated for something between 5 A and 10 A. They are more than capable of providing adequate current for flywheel blasters with stock motors, but might run into problems with electric spring-powered blasters or flywheel blasters with current-hungry upgrade motors.

In the unlikely event that players want to use cells with a different chemistry, ask them to show their cell's discharge rating - if they cannot, then it would be prudent to assume the worst.  

Battery packs are favored by players with more technical aptitude and concern for performance. Such players will want a pack with a good discharge rate for the sake of performance, which is more than enough to ensure that the pack is not overloaded. The packs that you are likely to see are intended for use in RC vehicles and are designed to support very high currents. It is therefore generally not necessary to manually check the discharge rating of a pack.

If you do need to check the discharge rate of a pack because it is concerningly small, this is easy. Lithium battery packs virtually always have the discharge rating and capacity written prominently on the side. Discharge ratings are given in units of C, where 1C is the current which would discharge the pack in one hour. For example, a 20 C 2500 mAh pack would be drained in one hour by a current of 2.5 A, and can sustain a current of 50 A, which is far more than enough for most blasters. To quickly mentally calculate the discharge rate in A, take the capacity in mAh, move the decimal point three places to the right, and multiply by the rating in C. Batteries also have a burst discharge rating, which is higher than their normal discharge rating and measures the amount of current that it can produce over a burst that is several seconds long. If only one discharge rating is specified, it is the continuous, not the burst, rating. The burst discharge rating of a battery is not important for safety (although matching the burst discharge rating of the battery to a motor's inrush current is generally recommended for performance).

In the unlikely event that a player shows up with a pack that is intended for use in consumer electronics, ask them about the discharge rating - if they cannot say with confidence what it is, assume the worst. 

It should be noted that battery packs are generally safer than loose cells because the low-quality spring contacts on which loose cells rely can generate significant waste heat.


Electrical power system safety: Lithium, (hopefully not) damaged

Lithium batteries are susceptible to chemical damage in ways that do not lead to immediate failure, but which can cause a failure later during normal charging or use of the battery. Recharging a cell excessively fast, at excessively low temperatures, to an excessive voltage, or after it has been overdischarged will - I'm skimming over the details here - cause various metal ions to end up in the wrong places, where they accumulate as solid metal. This is generally bad for performance, and more importantly for our purposes, may cause internal shorts.

Overdischarge - letting the voltage fall below a certain level - is the easiest way to accidentally damage a lithium ion cell. (To clarify: this is not the same thing as overloading. Overloading is discharging a cell to fast, and overdischarge is discharging a cell too far). The minimum safe voltage varies depending on what margin of safety you want and who you ask. The minimum voltage requirement for maintaining the performance of a battery is more stringent than the minimum required for safety. RC hobbyists often recommend 3.5V standing or 3.0V under load. Low voltage alarms typically trigger at about 3.3V, which is the point at which about 98% of a battery's useable capacity has been expended. This small margin of error is intended to allow users of RC airplanes just enough time to land. The Green Cloaks err on the side of caution and require a minimum voltage of 3.4V standing (except for LiFe, for which 3.0V standing is the minimum requirement).

There are several ways that players can protect their batteries from overdischarge:
  • Have more than enough capacity: A battery is not at risk of running out if it stores more energy than a player might feasibly need before they can recharge. As a rule of thumb, an electric blaster consumes 1 mAh per dart fired, and a battery with a decent discharge rate will typically have many hundreds of mAh, so a player in a typical game will run out of ammo long before they run out of battery charge. It is not hard for players who require more capacity to find batteries that meet their needs. However, overdischarge is still a risk if players neglect or forget to recharge their batteries. 
  • Use a sophisticated charger: Some modern chargers will refuse to charge a battery whose voltage has fallen below a safe level. This is effective because overdischarge by itself cannot cause a thermal runaway; only a battery that has been overdischarged and then recharged is dangerous.
  • Use a low voltage cutoff or indicator:
    • A voltmeter provides an indication of when a battery is running low, as the voltage of a lithium ion battery decreases steadily as it discharges. However, this is only helpful if a player pays attention to their voltmeter. Voltmeters draw a small amount of current, so those low-capacity batteries which benefit the most also suffer the most from having one. Some players choose to use voltmeters that only activate when triggered (e.g. by an open jam door) for this reason. 
    • A low-voltage alarm will, as the name implies, sound an alarm when the voltage of a battery system falls too low. Low voltage alarms intended for use in RC vehicles are typically capable of monitoring the voltage of each cell in a pack individually and will sound an alarm if any one cells' voltage is low.
    • A low-voltage cutoff automatically prevents current from flowing when the voltage falls too low. In theory, this would be an ideal way to prevent overdischarge because it is utterly foolproof - but, in practice, low-voltage cutoffs are hard to find, especially with voltage thresholds appropriate for lithium ion batteries.
I recommend that you require players who use lithium ion batteries to have some form of protection against overdischarge, which could be any one of the above.

Mixing battery types - that is, placing cells of differing chemistries or capacities in series or in parallel - is very bad design. It can lead to cells forcefully overdischarging or charging each other in an uncontrolled manner. This becomes a safety concern when lithium ion batteries are used. (To clarify: having multiple battery types in the same blaster is not a problem so long as they are not placed in the same circuit. It is common to see e.g. a blaster with LEDs for aesthetics that run on a separate battery from the blaster's main power supply, or a fully automatic flywheel blaster with separate batteries for the flywheels and pusher motor.)

Lithium batteries that have leaked electrolyte, swollen, or are deformed should never be recharged or used. Such batteries have undergone a process similar the the initial stages of a thermal runaway, and this serves as an indication that they have probably suffered damage which could potentially lead to a full thermal runaway. How a player treats their batteries before and after a game is beyond your control if you are a game organizer, but you can do a lot for the safety of your game by ensuring that batteries that show signs of abuse (leaks, swelling, bent out of shape) are never used.

Physical damage can cause a thermal runaway immediately. Some packs are made from hard-cased cells that are welded together and shrinkwrapped; these are generally pretty resilient. Soft-sided packs sacrifice physical durability for performance; they require physical protection, such as keeping the battery inside of a blaster's shell. Even seemingly minor crushing damage can cause a thermal runaway and the outer pouches can be punctured accidentally. The space in which a soft-sided pack rests in a blaster should contain no sharp edges, and padding to prevent the battery from rattling around is highly recommended.


Electrical power system safety: Wiring


Rewiring a blaster means replacing the wire, typically with wire of a larger gauge and in a simpler circuit. A poorly-done rewire - or stock wiring retained in a blaster that should have been rewired -  may short, which will severely overload the battery. If a lithium ion battery is used, this is dangerous. Experienced modifiers will tend to wire their blasters well for the sake of performance, but inexperienced modders like to cut corners, and there are a lot of corners that can be cut when wiring a blaster. In areas where the risk of fire is high (such as forests) it would be wise to ask players with modified blasters using lithium ion batteries to show the moderators their blaster's internals. Opening the shell would be a hassle; pictures should be acceptable. The Green Cloaks requires the internals of modified electric blasters to be inspected for this reason. (They also hold modified electric blasters that do not use lithium ion batteries to the same standard, but this is because blaster failure sucks even if it isn't dangerous.) Most games of HvZ do not require the internals of any modified blasters to be inspected.

A safe rewire should be both physically sturdy enough that parts are not at risk of coming loose, and have a low parasitic resistance. Parasitic resistance generates waste heat, which can cause components to fail and increases the risk that joins will melt and come loose. (Considering that parasitic resistance also harms performance, and that an inexperienced modder will often use a higher voltage to compensate for this, the waste heat generated can be very large!) In order to ensure this, a rewire should have:
  • Soldered joins: This improves both the quality of the electrical contact and the physical sturdiness of the join.
  • Insulation on electrical joins between wires: Joins between wires can move because wires are flexible, which can bring these joins into contact with other joins. Heatshrink tubing is ideal. E-tape and liquid e-tape are acceptable under the Green Cloaks rules, but beware that e-tape can come undone. Hotglue can come loose on a hot day or when a part heats during use, and is not good enough. Insulating joins to motor terminals and microswitches is recommended but is not always feasible.
  • Good wire routing: Crimping wires or trapping them between bulkheads leads to their insulation being damaged, and therefore can cause shorts. 
  • Decent wire gauge: Wire has an associated parasitic resistance, which is higher for thinner wire. I recommend erring on the side of caution and requiring 18 AWG - or wire that it thick enough to look like 18 AWG - in any blaster with either motor or voltage upgrades. (The precise gauge of a piece of wire is difficult to determine visually because the thickness of the insulation can vary.) Lower gauges correspond to thicker and thus better wire.
  • Electronic lock removal: Electronic locks have the potential to cause a short because they are wired to implement electrical braking when triggered, and the extra wire used to create the braking circuit can provide a path that bypasses the motor(s). The cheap microswitches that Hasbro uses cannot reliably handle high currents. Removing these locks removes a possible failure mechanism that could otherwise lead to shorts. 
  • No conductive debris, such as solder blobs formed from accidental drips during soldering.
While thoroughly checking a blaster for wiring flaws would be a great hassle, it is possible to tell a lot about the safety of a blaster at a glance. People who cut one corner will tend to also cut several more.

It should be noted that a well done rewire makes a blaster safer than it would be had the stock wiring been left intact. Stock wiring is often of a pathetically tiny gauge and various quality control issues are not uncommon with certain blasters. I, personally, have seen a soldered join in a stock blaster fall apart when the blaster was jostled. Properly made solder joins should never do this! Furthermore, a rewire will virtually always include the removal of electronic locks - because bypassing them is easier than soldering them into the new circuit - which could otherwise cause shorts.

Microswitch selection is important for reliability, but generally not for safety. Aside from electronic locks, the microswitches that must endure the most current are trigger switches in electric spring-powered blasters and acceleration triggers in flywheel blasters. Unlike electronic locks, they cannot cause a short - they will either cause undesired operation or prevent operation of a blaster if they fail.

Further resources


In summary, blasters with the following modifications are as safe as stock blasters:
  • Integration
  • Ergonomic improvement
  • Aesthetic alteration (no "realistic" or weapon-like colour schemes!)
  • Lock removal (possibly excepting jam door and magwell locks; see below)
  • Rate of fire increase
  • The use of aftermarket darts with a soft tip and mass similar to Hasbro's darts, such as "koosh" darts
  • The use of NiMH (or NiCD) rechargeable consumer cells in place of alkaline consumer cells
  • The use of NiMH (or NiCD) battery packs
  • Electronic modifications that increase reliability (These have the side effect of increasing velocity in electric flywheel blasters - but not by much. A well-done rewire can make a blaster safer.)
  • Expanding air tanks/bladders in automatic pneumatic blaster
  • The use of electric air pumps in pneumatic blasters
The following can be (and often are!) safely used for HvZ, but might be less suitable for small children:
  • Blasters modified for velocities higher than stock, with a limit placed on how hard the darts/disks/etc. may hit. A chronograph is preferable, but a subjective test - shoot a moderator, and see if they say "ouch" - works well enough if this is not feasible. 
  • Jam door, jam door lock, and magwell lock removal (a very careless player may pinch their own fingers, which is painful but not not seriously injurious)
  • Other aftermarket darts (but a mass limit and/or requirement for a soft tip may be required for player comfort)
The following are safe so long as the people who perform the modification and use the blaster do so with competence and care:
  • The use of lithium ion batteries, with an adequate discharge rate for the blaster in which they are used, and with the appropriate precautions
  • The use of higher pressure systems in pneumatic blasters, built to the appropriate standards
The following might be banned out of an abundance of caution if the game takes place in an environment where risk of fire is high:
  • Any lithium ion batteries other than LiFe and perhaps LiMn
    The following are dangerous and should be banned in a typical game of HvZ on campus:
    • Homemade or modified ammo with sharp protruding parts or hard tips that hurt on impact
    • Painted or otherwise decorated blasters which could be mistaken for real weapons
    • Blasters that are modified to produce very high velocities
    • Lithium ion batteries that meet any of the following criteria:
      • Inadequate discharge rate for the blaster in which they are used
      • In combination with shoddy wiring that may short
      • Soft-sided packs without physical protection 
      • Has no protection against overdischarge
      • Has swollen or leaked
    • Modified pneumatic blasters with any of:
      • No overpressure release valve
      • Inadequately rated parts
      • PVC airtanks
    • Unsheathed protruding metal barrels 


    1. "...PVC pipes can be safely used with pressurized water because water is *incomprehensible*"

      Yeah, I don't understand either. ;-)

      1. *Incompressible. Typos are bound to get through occasionally.