MGC68 with Marushin mp40 cartridges

I just got a new lathe and for one of the first projects I wanted to make a detonator for the MGC68. While I do have a few p220 cartridges I have many more Marushin mp40 cartridges.

I did find drawings for both detonators. There are some differences in dimensions. Though some are rather small. The length is the biggest difference.

I decided to make the detonator to the p220 drawing. If it doesn't work with the Marushin cartridges I can make changes later. I also decided to make the detonator in 2 pieces. It makes it easier to machine and the pin can be replaced if needed. The part that fits in between the barrel and trunnion will be the same no matter what cartridge is used. I made the shank of the pin smaller and has a shoulder that fits up to the other piece.

After quite a bit of difficulty I ended up getting the part made. It ended up being a little off of some of the dimensions, but nothing is really critical. I used some super glue to join the pieces for some testing.

I removed the barrel from the MGC mp40 and dropped the detonator in. Then I found that the dimensions are quite a bit off from my barrel and trunnion. The detonator has quite a bit of wiggle room.

I loaded a cap in a cartridge and gave it a try. It did fire and ejected the cartridge across the room. While the bolt did cycle it short stroke. I tried a few more times and it went exactly the same. I'll have to look into that problem.

Since the cartridge did fire the project is kind of a success. Now I get to try again with dimension taken from my replica for a better fit. At least since I made it in 2 pieces, I can reuse the pin.

At first I was using the recoil spring that is just a coil spring. That didn't always want to fire.

Next I switched to the multi piece enclosed spring. That fired every time. Though it was still short stroking.

I tried a Marushin cartridge in the MGC68 and it didn't fire. Looks like the pin is too short.

I tried the cartridge in the Marushin and it fired, but it also short stroked.

I remade the detonator using dimensions from my spare barrel and trunnion. When I went to insert it into the replica I was planning on firing I found out that the hole in my spare trunnion is 10mm deeper. Also the casting isn't exact. After reworking the detonator I got a snug fit, but still easy to pull out.

I noticed that my bolt was jamming when it was against the trunnion. I added some lube and worked it back and forth. Now it doesn't jam.

I loaded up another cartridge and tested the new detonator. This time it functioned perfect with the bolt resetting.

After looking at how the detonators sit in the barrel it looks like the Marushin is closer to the breech. The Marushin's dished washer is shorter than the MGC's piston, but it still looks like the Marushin detonator is longer.

I'll measure the differences between the cartridges and make the pin longer. Hopefully that is all it takes. Otherwise, I'll try regluing the pin farther back until it detonates.

Hi Punisher,

It may well be that Marushin's MP40 / UZI cartridges don't contain enough gas pressure when fired. The screw on bases are known to leak gas after some time.
Bear in mind too that Marushin's ABS MP40 uses a very light weight plastic bolt. MGC's MP40 has a much heavier metal bolt so, chances are, the cartridge isn't up to the job you're asking it to do.

My own Marushin '40 is the first modelgun I bought nearly 20 years ago. It cycles superbly well even using Marushin cartridges I bought way back then.
The MGC '40 performs superbly well using MGC P220 cartridges.

I'm sorry to read you had problems getting a good fit with the Det. Pin. I drew a Det.Pin giving dimensions for my own model. I did state that models vary dimensionally and anyone machining one shouldn't take my quoted dimensions as finite. Still, I apologise for causing you some headaches!