Making MGC Thompson cartridges

I have a MGC Thompson, but no cartridges. I'm hoping that I can make some.

I tried a fired .45acp case and it fit the Thompson bolt. It's slightly smaller than the cutout in the face, but the extractor holds onto it. It's loose in the detonator, but the pin should guide it in. It might need a little bit more of a taper on the cartridge's hole.

I'm thinking that it should be possible to 3d print the rest of the cartridge. With the right sizing it could be press fit into the .45acp case.

The first test was a complete failure.

I started with the tip rounded similar to the MCG cartridge. When I tried feeding it would hit the feed ram and jam. It didn't even clear the mag lips.

I decided to give it a test fire with the cartridge inserted into the bolt. A cap was loaded all the way at the back. When I pulled the trigger the bolt went forward and nothing happened. The detonator pin wasn't long enough to reach the cap.

I tried again with the cartridge in the chamber and another cartridge inserted in the bolt. This time the cartridge was able to go deep enough to set off the cap. It went off, but there was no attempt at recoil.

My Thompson was a non firing model. The new firing bolt that I got has been jamming in the receiver.

I decided to work on the feeding problem first. I also switched back to my original bolt. It has a full face, but had the dummy extractor. Previously I had welded on a piece to make it function.

The front of the cartridge is now shaped more like a bullet. That alone almost fixed the feeding issue. It would still jam about a 1/2" before being in battery.

I adjusted the opening and added a taper on the inside. I've tried it several times and it's feed reliably.

I slightly adjusted the scale of the model in the slicer to get it to a press fit. I'm not sure yet if just a press fit will be enough. If it's not it could be ran through a reloading die to put on a slight crimp.

The feeding does slightly damage the tip. A slight change might fit it. It should be good for a few fires as long as the press fit idea works. If not it's under 2 cents to print one so being single use isn't too bad.

Good to see some thought and ingenuity put into practice!

Fitting a 7mm dia internal piston / 0ring seal/ firing pin as per the MGC design should give the blowback power needed to cycle the bolt.

Good luck with this, keep us posted please

Looks good and i am with Cerwyn withthe piston and seal, good luck.

I would like to avoid using an o-ring. The multi cap cartridges don't use them. If I'm understanding how they work right the front cap forms a gas seal for the stroke. Then at full stroke a hole is blown through it.

I did another test with 2 caps facing each other and an anvil in between. This time there was a slight start at recoil.

I thought that maybe the inside wasn't smooth enough so I reamed it. I tried again with 3 caps. That was pretty loud. There was about 1/2" of recoil.

Thinking about it the problem was probably that I didn't have anything in front of the front cap. It blew forward onto the detonator pin and caused jamming.

I noticed that the rear of the printed bullet was blowing apart. It's probably mostly because of how I was printing it. I've been printing with the extrusion width bumped up to .6mm for stronger prints. That caused the model to slice with only 1 parameter at the base with a slight gap in between.

I resliced with a .4mm extrusion width and only 1 parameter. It looks that that will make the base solid. I might have to bump up the extrusion multiplier to try to over stuff plastic. I'm also going to try making the hole less than 7mm and ream to size.

I have a Marushin mp40. It happens to be the only model that I have cartridges for. They don't use an o-ring so that's what I've been trying to copy. I loaded one up and it functioned perfectly.

I tried reaming the printed bullet, but it didn't seem to make a difference. Reducing the id and reaming didn't do any better. It might not be possible to get a 3d print repeatedly to the same level of fitment. I ordered a stainless tube to see if it would help.

I ordered some o-rings to try out. It might have enough flex to seal the imperfections. I was thinking of trying the cap facing the rear with an o-ring piston. That should give it 2 chances at sealing. It turns out that how the Marushin .45acp works. Thanks for posting the drawings Cerwyn.

One of my test built up enough pressure inside to almost blow out the primer. I had removed the primer from one case and modified a m3 socket head cap screw to fill the hole. The screw protrudes the case making an anvil like the Marushin. M3 was as small as I had and required reducing the od of the head and grinding it down after it was installed. M2.5 should fit better so I ordered some.

It's going to be a week before I get the supplies. Even with the added pieces and buying once fired brass it should still be under $1 a cartridge.

I inserted a Marushin mp40 cartridge into a .45acp case. It ended up being 32mm long. Even though it is slightly longer it still fits in the mag.

I printed out a spacer to hold the two together. I loaded a cap and assembled the cartridge. It fired and ejected. Unfortunately it only blew back far enough to eject. Not enough to recock the bolt.

The cap sealed enough to push the Marushin's plastic bolt, but not enough for the MGC's zinc bolt. Looks like a piston with an o-ring will be needed.

Interesting read Punisher. Keep on going

Some of the parts came in today. The 5mm long m2.5 socket head screw almost worked.

The case can be tapped without drilling the hole bigger. The head of the screw fits the primer pocket, but still isn't flush. With a little grinding it works.

I measured the protrusion inside the case and according to the Marushin cartridge it could be another mm longer. Luckily 6mm long screws are available. Switching to a button head screw should make the head recessed inside the primer pocket.

I tried printing a piston. I had to modify the design a little so that it could be printed with a fdm machine.

With the o-ring dry it was hard to push the piston inside the printed bullet. With some lubricant it slid back and forth easily.

I did another test fire and it didn't work any better. When I pulled the cartridge out I saw that the bullet was pushed out of the case. It was a tight fit that required a vise to fully seat.

I have some p220 cartridges coming. Hopefully with swapping parts around I can figure out what's not working.

The good news is that even though the screw is a little short it still fires the cap.

It does sound as if printed internal surfaces inside the cartridge are causing too much friction, stopping the piston's free travel.
When the P220s arrive, compare piston travel / stroke length to the drawing of MGCs 45 cartridge I posted for you.
That length of travel is vitally important to give enough blowback power to cycle the bolt. Likewise, sealing expending gas pressure inside the cartridge is vital. That's one reason why cartridges are threaded together, not just for ease of assembly. As brass cartridges wear, very noticable with P220s, the threaded portion of the top spreads outwards. The result being, eventually, leaking gas pressure and lost power.
Marushin's MP40 cartridge suffers from this too.

The p220 cartridges came in. I loaded one up and inserted it into a .45acp case with a 3d printed spacer to hold it in. It fired and ejected, but didn't reset the bolt.

That was a bit disappointing. I was really hoping that it would be a working option.

I tried the p220 piston inside the 3d printed bullet reamed to 7mm. It fired, but didn't recoil. When I ejected it the bullet was almost pushed all the way out of the case.

It's looking like a press fit isn't going to work. I got in a .45acp reloading die set. Next I'll try putting a crimp on the end of the case. That will make disassembly harder, but I have a couple ideas to try.