Experimental T19 belt-fed conversion overview

Now I have a machine gun. Ho ho ho.

Belt feed has always been an underutilized technology in the sport, and I have always both seen the practical value in it and just liked it. Back in the HvZ glory days I had a Vulcan GPMG setup. Anyway... Best to go back to how this line of development started.

This was an instance of quarantine-induced BoredomCAD that occurred:

The idea that perhaps disintegrating rigid belt links would work with caseless foam dart ammunition has often been thrown around, sometimes experimented with, frequently rejected, occasionally reported to work well, but it just hasn't really gone anywhere out of any of those ventures, so I decided to take a whack at it. At this point, with full length darts.

What do you know, it sure does seem to work nicely.

At this point, with the link design roughed in, short darts become involved.

There is particular interest in using short darts for belt-fed platforms. Smaller ammo and easier carriage is obvious, but another reason is the potential of belt feed as a practical way to use short darts in a full auto platform that isn't a box magazine, given the reliability history of the latter with short darts in this locale.

 

The shorty belts proved satisfactory as well. Now, I should note the intent never clearly existed at first to make a blaster to feed these belts just yet, but by this point, the situation was screaming for one to exist as soon as possible.

Given the highly experimental nature of this first testbed, I opted to modularly convert the T19 platform by adding an independently driven and controlled sprocket feed module as the best way to evaluate the belts without wasting a huge amount of time in the arms race or a huge amount of effort digging a hole that might just be in the wrong place by designing a new, tightly integrated blaster.

The sprocket is direct-driven by its own NEMA 17 motor inside the breech module. The standard T19 drivetrain stack handles the bolt aspect. A standard S-Core controller would do flywheel motor management and bolt drive while communicating to a slave board, then and now just a second S-Core, managing the feed sprocket.

In such a way linkage designs associated with pawl feed and single drive are avoided and the number of CAD/print iterations required to debug and spin up this design reduced in favor of software brute force, while also providing an extreme degree of controllability over stuff (like belt feed acceleration) that just isn't possible or easy post-build with linkages, and allowing features like restriking stuck rounds without attempting to feed the belt which have proven very useful against malfunctions. It comes at a cost and also, at a mass, as well as some limitations on cyclic rate of fire due to the speed and acceleration limitations of both drives, but for the nature of this it is a useful approach - and it fits the most basic MO of software-defined blasters overall as well which is a very successful one.

 

IT'S ALIVE!

The test mule went together well, the code came together quickly and was cycling and (mostly) successfully firing ammo the same day, and it was off to the great bug hunt. This slow motion clip I took for evaluating timing issues.

And just a bit of hacking later and this is obtained.

Attention then turned to hammering out all the mechanical and control gremlins. The trailing edge of the feed sprocket slot in the feed tray was causing a snag, so the ramp got ground in by hand, and then once tested, CADded, along with dulling some other trouble spots.

I initially did not have this green sharp edge there - the feed ramps were one radially symmetric feature everywhere except where the tray intersects. However, this gave poor constraint of the link being stripped at the time, with it potentially rotating forward in the XY plane, causing the projectile to also yaw and be mashed into the feed ramps, and also potentially being itself stuffed inside the feed ramp and then snagging on the next feed. The feed tray cover holds the links down onto the tray, and the inside surface of the link bands is lower than this edge (while the top surface of the band is NOT so), so it stops the links in the forward direction without interfering with the projectiles.

And the tray cover was fitted with a spring-loaded pressure plate over the feed tray. This removes backlash from the belt and sprocket caused by a fixed clearance there and adds damping as well to eliminate bouncing of the belt links inside the tray on each feed.

Further refinements were all on the software side including fully closed-loop cycling with no preemptive timing at all, automatic restrike without feeding on failed bolt cycles, and dialing in the motor drive parameters.

With that, the testbed was torn down and construction commenced on the field trials blaster. From cyberspace...

...To reality.

Haha, Prusa go brrrrrrrr.

And... So does the blaster of course.

After tying down loose ends, fixing some CAD goofs and finalizing the configuration (slightly shorter barrel, revised bipod parts, no more iron sights, and Picatinny) it's pretty much done. The rest of this post is a build album and some firing videos of the final configuration, so to get this out of the way:

Release Link

 

Of course, for anyone who's about to click that, this is still highly developmental and edgy stuff, and it's a sprawling mechatronic mess of a build. I might have the control gear boiled down to something canned that I can sell you in the future, but for now yeah; there be dragons, you should be confident working with software-defined blasters or something similar like building non-kit printers to tackle this one. I have preliminary notes in there that cover a bit more about the build. Unfortunately I didn't get any WIP images on this one, but I will work on documenting them with CAD renders instead and definitely have an opportunity later to document another real build of one.

This is Luna. She likes firepower. Every time I put this thing down on this bed on the way outside to do a test, she comes along and flops by it or on it.

Note that skid mark on the white feed tray - That's from dart tips as they are fed. Guess my alignment is set right on the money then...

Well that's just about that. Unfortunately, it still looks like a live field test is a ways off.