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 Air Lock in Cartridges...

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Cerwyn
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PostSubject: Air Lock in Cartridges...   Fri Aug 15, 2008 5:43 pm

Post by Mark

I notice from time to time that I get cartridges that refuse to fire. At first I thought that it was the cap but apon disasembly, I discovered that most of the time the cap wasn't even touched.

The problem, I now know, is possibly caused by an airlock situation due to trapped air by the cap. Because the O-ring doesn't allow trapped air to escape and this acts as a cushion that effectively stops the piston from hitting the cap. Especially when the bolt or the slide is fairly light weight. The airlock effect can be somewhat alleviated by the proper use of silicone or grease.

Also, the Hudson piston has a air bleed hole that will eliminate the trapped air. But, the detonator needs to match the piston style closely so that none of the initial gas pressure is lost.

I have had good luck by "pre-setting" the piston by pushing down the piston so it just touches the cap. This will usually expose a cartridge that has trapped air when the piston is "spongy" or springs back.
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MGC MP40 Marushin MP40 Nakata MP40 TRC MP40

Modelgun manufacturers, past and present:CMC,CAW,Hudson,HWS,LS,MGC(R.I.P. 1960's~2007) Marushin,Marui,Kokusai,Shoei,Tanaka and Western Arms
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Cerwyn
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Number of posts : 10633
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PostSubject: Re: Air Lock in Cartridges...   Fri Aug 15, 2008 5:43 pm

I spotted Mark's comments about airlocks within cartridges...
I've experienced this once or twice with 9mm mgc rounds in paticular.
Assembling the round, greasing the pin's o-ring and attempting to place it into the case will see it pop out again as if pressed hard against a spring.
Teasing the thing back out fully and pressing it back in again often solves it though.
I was given a tip when i bought my first Thompson (MGC, 1921). On assembly, push the pin and o-ring all the way in until it almost touches the cap. This worked fine, it's necessary anyway if you use a 7mm AND a 5mm cap at the same time otherwise the 5mm cap falls away and probably will turn sideways resulting in a total missfire. I was using 2 caps simply because I wasn't lubricating the rounds well enough. A standard 7mm cap didn't have enough kick to push the dry internals back, 2 caps did, but obviously proper lubing means the round works as it should with a single cap.
Another problem with airlocks happens with marui rounds. These are one piece rounds, having the detonator pin cast into the base of the round. Caps are pushed down into the round, charge side facing the pin. There's no other means of sealing or closing the round so the cap does everything. To achieve blowback and automatic ejection and reloading, the cap has to seal tightly otherwise the gases leak, silicone grease is a must to ease the cap down the round during loading and to lubricate it's travel backwards after firing but the grease and tightly fitting caps can create airlocks which lead to regular misfires.
WD40 or similar seems to work slightly better but it's really difficult keeping the charge dry.
Cerwyn
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Cerwyn
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Number of posts : 10633
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PostSubject: Re: Air Lock in Cartridges...   Fri Aug 15, 2008 5:44 pm

Post by Mark

The MGC, Tanaka O-ring design inherently traps air as the primer and the cap seals the bottom of the cartridge and the air that was trapped at the top of the cartridge when the piston is placed on the case mouth. This trapped air is sometimes difficult to bleed out. The primer can be pushed in a little to re-leave the pressure that the trapped air creates as the piston is depressed.

When air is trapped the detonator cannot usually strike the cap with enough force to detonate it. This causes many of the fullauto firing problems that I have experienced as the trapped air slows down the somewhat lighter bolts (like the ABS MGC M16 series) to the point that they stop firing. This is a problem that can be somewhat alleviated by hand cycling the loaded cartridges through the action once before firing them.
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Wanted to buy: MGC Sten MK3....trades?(still)

MGC MP40 Marushin MP40 Nakata MP40 TRC MP40

Modelgun manufacturers, past and present:CMC,CAW,Hudson,HWS,LS,MGC(R.I.P. 1960's~2007) Marushin,Marui,Kokusai,Shoei,Tanaka and Western Arms
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