Thursday, June 11, 2015

Tacmod 3.1 Completion (contains rambling)

Well, it has been a long time coming and a lot of hard work, but as of one week ago to this day, the newest member of the Tacmod family is finished.

Another shiny new Noob Dream Rifle fresh off the tech bench.

This is one HEAVY, SOLID gun. I haven't weighed it, but if I handed it to you, you would be surprised. Especially with a typical setup of accessories it is really an extremely confidence-inspiring piece to wield.

Nothing unexpected about the setup. Ever-present eotech, and Convoy M1 light.

Having the latter mount properly, perfectly straight, and rock solid with no duct tape is so welcome.

No sling mount solution has been devised. I will probably have to make a bolt-on steel clip loop out of some angle by hand because I have searched high and low and the only existing part I can find that does what I need is a Magpul Masada sling mount and those are scarce and expensive.

You can see that I didn't fudge up the pusher endbell cover this time, I fabbed, installed and smoothed it and THEN painted. It hides a lot better that way.

The rocker switch completely disables the pusher circuit, which is used as added safety since the pusher circuit has the potential to operate randomly by itself if a switch fails (and possibly smoke the motor, flatten the battery or start something on fire). I opted to omit the arming plug, because arming plugs are a bit unclean on the outside, and that is one more contact to fail or get dirty anyway. Thus there is no making this gun inert while it is assembled for the time being. That rev button is always live.

More headscratching will be done on that. I might try to velcro the arming Deans on top of the pusher box, under the jam door, so you can lock it out for transport.

Speaking of the jam door, this is the not-quite-faux charging handle, showing the very non-aesthetic reason it was added.

Magnetic latch is a 6x1mm neo Devconned into the receiver and an aluminum standoff with a magnet on the end (surplus) Devconned into the jam door. It is easy to open but won't flop around loosely and stays closed if pointed upward.

Toploadability is in fact retained. It isn't quite as easy as a carry-handled RS but is not a problem. While guarding areas in HvZ you can pop rounds in and keep the mag full.

This is a better cleaner image of the battery:

For non-readers of DZ, it is lithium-ion, 7.4V 1600mAh 19C, and the cell type is Sony US18650VTC3, typically found in power tools. More or less, it's a LiPo battery on durability and safety steroids. I have specialized equipment to assemble these packs and they are what power my whole arsenal.

Since I first posted an image of this pack, I removed the original shrinkwrap because it split during shrinking, cleaned the messy tape residue off one of the cells, and added a piece of insulating board between the center interconnect tab and the positive end of the cell. This is extra insurance to prevent the tab from wearing through the cell shrinkwrap and insulating washer and shorting should it end up moving or vibrating for any reason. I then rewrapped the pack.

Battery Detour

I have also found some highly interesting information about the R-series Samsung cells. The Sony VTC series with its widely publicized 30A rating has received all the hype in the world of high-current 18650 cells, mostly from the E-cigarette crowd. The Samsung INR18650-20R and -25R, meanwhile, have a datasheet spec of 22A continuous discharge. This causes or caused both me and most of the vapers to steer clear of them when there are cells - and sellers - boasting 30A in huge neon letters.

However, ratings are just not that simple. The continuous ratings for these hybrid cells are based on operating temperature and cycle life when cells are lugged continuously at high current all the way to empty in one shot. The manufacturers pick a capacity loss threshold to consider "worn out" (say, 80%, or 60%) and a minimum cycle count (200-400 typical) and then specify a continuous discharge current to achieve that. It doesn't really reflect actual use on actual loads that are much more transient. Also, the ratings produced vary by not only cell IR and current handling capability, but by what testing thresholds the manufacturer decides on which are not the same between manufacturers; and the high-temp durability of each cell's particular chemistry.

The Sony does not have superior IR to the Sammy, it does not hold voltage under (even 30A) load as well as the Sammy, and it is more expensive. The testers have beaten the first two points into the ground, and the market has spoken on the third. Plus, Samsung is the industry leader in power tools. The 20/25R are used in some very serious applications. Their actual burst current rating is a whopping 100 amps for 1 second. That is superior to the Sony and all others I am aware of.

If the overhyped green cell's coffin needs any more nails, it has to be that the 2500mAh top of the line -25R cell is just as cheap as the outdated 1600mAh VTC3. So next battery build will be getting away from Sony.

Now back to the T3.1. Here it is wired and assembled.

This is the "enhanced duration" cycle control follower.

It is fabricated from PVC and styrene, forming a pocket for the stock flat lever actuator on the Omron SS-10GL switch. The design is based on the BSUK printed component.

Changes from the BSUK part:

  • The length is increased to take full advantage of available room. This plus the tip profile is what gives the duration boost; with this follower, the switch cuts to braking earlier in the retraction stroke and keeps braking longer into the start of the extension stroke than other styles, giving a longer window in which to arrest the motion of the bolt before an overrun occurs.
  • The profile, viewed from the muzzle end as installed, is trapezoidal with the top being thinner than the base - whereas the BSUK part is rectangular. This chamfer prevents the follower from snagging on the gearbox shell resulting in a malfunction that was sometimes reported by BSUK FCG users.

With the XP180 pusher motor, ROF varies between 10 empty and 12.5 fully charged. Not yet audio-verified, but this is a fairly fast gun, faster than my other two.

The pusher circuit is wired classically (live-center) with no drop diodes. This follower and the neo-magnet XP180 motor give absolutely phenomenal control and trigger response. I highly recommend the Xtreme Pro180 as a pusher motor.

Well... now let's get to where I don't like the XP180.

Which is on flywheels!

I was really not impressed.

Sure, they shot just like FKs, and they had response, perhaps a hair better than FKs, but they are nothing like the smooth, peppy, quiet (on 2S) and all around professional and rock solid FK180SH-3240. Instead they run very "stiff", very rough, and very loud and just feel very "flat" and unhappy. Those neo magnets drag pretty bad when coasting down and I suspect are the primary cause of the general roughness. Overall they are not pleasant to shoot.

Out of all motors I have worked with, they most remind me of the Sagami 280 that I used on a Stryfe conversion years ago. They sounded very similar.

The real hard problem I ran into was speed matching. FKs are generally damn good about it, and if you keep the flywheels balanced and the buildup doesn't get uneven, they stay in really nice harmony and shoot very accurate. These things were not so. I had to switch one of my fly motors with the pusher to get it matching better and even then it was not great. I only have 3 and they are not a cheap motor so there was nothing further to do. They tended to wander with use, too. That to me is a dealbreaker; I would have to purchase a ton of them, fully break them all in, and then match them by audio to get FK-like speed matching results.

In the end, the Tacmod 3.1's flywheels were summarily switched to FK180SH-3240. Wow, what a difference. MUCH, MUCH nicer.

And around here was when I found myself in the midst of a nightmare of the alignment variety.

I started with a Stryfe cage, and feed guides installed. Velocity was low and squibs happened. Pulled the guides (they were an experiment), both improved marginally. Accuracy still garbage, and shooting downward. Swapped to a RS cage and the new FK motors, no change. Tried some feed ramp changes, cleaned the inner barrel, examined the muzzle device for contact with darts, all to no end. Tried cleaning, mold-code matching, and swapping different sets of flywheels. No dice. Made up an inner barrel alignment tool from a stick of tubing and checked that. Perfectly straight install, nothing to fix...

I eventually figured out that something was NOT RIGHT with the cage mounting bosses in the base Rapidstrike. I had a sideways offset in conjuction with a Y-axis angular misalignment between the cage and the inner barrel. This was why a brand new custom gun shot like shit. Barely perceptible off dimensions. The cause: The receiver was warped. It has some visible external wonkiness if you know where to look. Indeed, all RS have it, even my other 3 nearby - but this was a particularly bad one and it affected the internals.

God dammit Hasbro, not another lemon RS. You know if I am hitting the lemon lottery this often, that the public are too! Fix your shit!

Well, thankfully I salvaged the situation. The cage is now shimmed out a half mm on the bosses, with a couple mounting ears de-counterbored and a few bosses ground slimmer, and wedged into rotational position with a piece of PVC, and it shoots great.

In the end of it all I don't know if I have ever said this, but you know what, I hate teching flywheel guns. Dread it because of these nasty insidious issues mostly revolving around geometry and part tolerances. There is nothing worse than having a project land in this limbo and you never be sure whether you or the project and all the hard work you put in will ever escape or if you will be stuck with that failure.

I get sicker and sicker of quality issues on every project. I figure it is symptomatic of the same going on with the larger hobby. Sooner or later something is going to have to give. We need to remember modding toys is a stopgap and always was.

Random rail install closeup

At least another development came of all the fix attempts:

I gave Rayven flywheels a try.

They are nice and heavy with thick rims and are always so nicely balanced and smooth. The problems have been with shaft fits. All the ones I have handled in the past were what I would consider too loose for 180 service. However, I had a set sitting around, and you know what? They fit fine. Just as tight as all the Stryfe fly sets I also tried!

They are in there now, and they are very nice and quiet. I cleaned the shafts with alcohol. I am not worried. I have shot a good few thousand on them so far and used them in the FL heat in this gun's first trial at this past TBNC HvZ woods game.

Which went excellently, by the way.  Not much to say there - like clockwork, it was. Solid state.

The velocity and accuracy are continuing to come up with use. With the higher ROF it is fun as hell to shoot with koosh and the eotech. Put the reticle on something at 35 feet and rock it with a directed foam hurricane.

It's just gonna take a while for me to stop being shaken by that alignment fiasco and start being excited about my new primary.


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  2. Hello,

    I recently purchased a used stock rapidstrike and I'm experiencing what seems to be some of the same issue that were had in your troubleshooting. It'll fire a few darts but most of a full mag will barely escape the barrel. This is my first rapidstrike so I'm not sure what is causing the issue or if it will remain when I overhaul the blaster. I would greatly appreciate any help or advice you could offer on what might possibly cause that or how to possibly fix it. Thank you for your time

    1. Minor alignment issues usually show up only at high velocity. Start with the most likely explanations; are both the flywheels turning?

  3. How does that charging port work? I saw your internal with the deans running to what I would guess two of the pins, but what about the port plug ( the white ended one that conveys the battery's info to the charger?)

    1. The Tacmod charge connector is 5 pin and carries all 3 isolated balance taps (for the 2 cell battery) plus the 2 pins for the charge current.