Marushin Cartridges causing UZI Firing Problems

Post by Madmike

Well I had an old Marushin kit UZI sitting around for over a year and I finally managed to assemble it. It came with five rounds too, so I tried firing it, which worked rather well by using only one 7 mm cap and no other things like used caps as seals or another 5 mm cap.

So I decided to get more carts, the current MP 40/UZI carts. They look exactly the same like the carts that came with the gun, but they don´t work! Everything is the same, gun, detonator, caps, but it does work with the older carts, but not with the new ones. They ignite but don´t have the power to push the bolt back, and they are very quiet. So I guess they loose the pressure somewhere, maybe at the threads. This is really strange as i can´t see any difference between the old and the current carts.
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Post by shazhib

madmike,

what about an inner piston? is that different?

Post by Madmike

It looks the same, but I could not yet take exact measurements, maybe the new inner piston is a little smaller?
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Post by shazhib

OK,

My suggestion is:

1) Use a used cap for sealant
2) place donut shape rubber sheet, maybe 0.5mm thick, onto bottom of the rounds
3) If neither 1) nor 2) works, consider converting it to MGC SIGP220 rounds usuable version.

S

Post by Madmike

Okay, thank you very much! I am really not happy as the gun seems to be all right, bot the newer carts don´t!
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"It's a hardball world, son. We've gotta keep our heads until this peace craze blows over."

Post by Madmike

Well, I tried the "used cap upside down as a seal" trick, blowback is stronger, but the bolt is still not pushed back far enough. It must be the new carts, the gun operates flawless with the older carts. I just can´t believe that...
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"It's a hardball world, son. We've gotta keep our heads until this peace craze blows over."

Post by ljerr2

Is it possible that the newer carts seal too well? Maybe the friction is too great between the newer piston and wall? Probably not if you felt greater recoil after using a spent cap for a seal, but it might be something to think about.
I tried different methods with the Marushin round - spent cap, double capped, etc but had the best luck with converting to the P220 MGC round.

That's a strange one!

I've always found the Marushin 9mm MP40/UZI rounds work perfectly in my MP40.
I make sure the single 7mm cap has a thin smear of light grease on the edges and so far I've not had any problems at all with them.
Even though every cap is punctured by the rounds firing pin they still produce enough blowback to cycle the bolt and it will fire a magazine full without stoppages practically every time.
The MP40 has a lightweight plastic bolt though so wouldn't need much effort to push back I guess.

The UZI I have is all metal and was converted to fire P220 MGC rounds before I bought it. The bolt and carrier is all metal and much heavier.

One modification or Tuning tip shown on the Instruction leaflet is to cut the recoil spring which should allow the bolt to cycle easier.

But this doesn't explain why the original Marushin rounds work but new ones don't.

Does the cap get pushed away from the rounds' firing pin after igniting or does it stay attached to the pin?
If it's sticking to the pin it won't push the piston away from the gun's det pin for blowback.

Cerwyn
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Post by Madmike

Thank you for your answers. I will try using grease inside the carts. I have a feeling that the piston is slightly to small in diameter with the new carts, but I may be wrong. The carts ignite, but the blowback is too weak, they must loose their gas pressure somewhere. What really upsets me is the fact that the gun seems to be fine, working really good with the original five carts, but not with the newer ones (that should be the same). Marushin must have changed something, or maybe the tooling is worn now after the years, thus more tolerances, I don´t know.
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"It's a hardball world, son. We've gotta keep our heads until this peace craze blows over."

Hi MadMike

Just had another thought, have you tried the new pistons in your old rounds?
Or try the end caps and firing pins of the old rounds on the new bodies?

Any difference in performance then?

Cerwyn
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Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.

Post by shazhib

Cerwyn,

I think that is really good point. Inter-change inner piston and tail of cartridge may identify the problem.

Other thing just came across to my mind is, check if there any hole on a center of the cap after you fire. If a firing pin in a tail of cartridge is slightly too long, it may make a clear hole on a center of the cap which end up letting the gas faster than old cartridge you have.

It is so strange, but small trivial setting of cap and cartridge sometime makes a significant impact to firing performance.

S

Hi shazhib

Even though my Marushin MP40 rounds puncture every cap fired (I've well over 100 by now and fill 3 mags at a time) they are still well able to generate enough blowback to cycle the lightweight plastic bolt without stoppages.

Could it be that MadMike's UZI needs the extra power a non-pierced cap would contain to cycle it's heavier bolt?

You mentioned a rubber disc earlier, would this be placed between the rounds' piston and cap acting as a seal even if the cap is pierced?

Cerwyn
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Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.

Post by Mark

Cerwyn,

The rubber gasket is meant to seal the threads at the bottom of the Marushin Uzi/MP40 cartridges. The gasket is small enough to fit in the base and yet large enough to seal the base of the thread form when the cartridge(s) are assembled. The main thing that comes to mind to me is the lubrication in the cartridges and the Uzi itself. I have been successful firing many modelguns with the Uzi/MP40 cartridges and they seem to blowback quite heavy bolts with out either the rubber gaskets or the fired cap as an extra seal. I have to say that the main reason that I have had good luck is the fact that I lubricate the inside of the cartridges with grease.
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MGC MP40 Marushin MP40 Nakata MP40 TRC MP40

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Hi Mark

I have to say I agree with you about the effectiveness of the Marushin MP40/UZI cartridges. I don't use any additional gasket nor found a need for inserting dead caps when firing my MP40.

A good and thorough cleaning inside and out, a smear of grease on the cap's edges and that's it.

My MP40 will consistently fire through a mag loaded with 30 without stoppages.

I've always been surprised when reading about poor performance from these rounds and the soonest a modelgun is converted to use MGC P220's the better. It must be said though that I don't fire them very often. If I do shoot it's 90 or more rounds loaded into 3 mags rather than a quick re-load of 30 and fire again.

As mine get older they may well start to leak about the threads etc and then I'll need added seals.

I've almost finished my debut video of my MP40 so I'm trying to figure out how to download and all that stuff now. Hopefully you'll see how well these Marushin rounds go

I'm trying to get my UZI firing consistently at the moment but it's already been converted to P220's

It is strange that MadMike is having problems with 2 different batches of rounds though. The only difference I've noticed when buying new rounds is the exterior casing is machined very slightly differently but that's only aesthetic and certainly shouldn't affect firing.

Cerwyn
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Hobby collector of mainly WW2 era uniforms, headgear, field equipment and replica weapons.