The Marushin Beretta center fire series of cartridges all seem to use this 4 piece design, it is a functional design (albeit, not a truly good design in my opinion, when compared to the other manufacturers designs.) Although, it is considerably better than the earlier side fire 3 piece cartridge as it employs O-rings at the cartridge separation point. This helps to control the gasses helping to utilize them for the blow back effect.
Marushin seems to like putting the separation point low on the case perhaps this is for manufacturing considerations? or copy right restrictions? I am not quite sure, but this weakens the case as the fact that the cap is 7mm and the cartridge case outer diameter is probably 10mm?(9.5mm) leaving 1.2mm? of wall thickness at the threads. And at that point, the threads are highly stressed due to the high gas pressures at that point, adding a O-ring helps prevent the gasses from escaping through the threaded point as these threads are straight cut (not pipe threads) However, adding an O-ring at this point also raises the pressures, practically ensuring case failure at the threaded point.
The piston in this case, also features an O-ring in the middle of it's outer DIA which also helps retain the gasses helping the blow back simulation these are a step in the right direction, compared to the older side fire cartridge that relied on the cap body to seal in the expanding gasses.
In fact, there is a double gas seal in this area due to the O-ring and the cap body, which again raises the gas pressures in the case.
But, the friction of the O-ring and the cap body against the cartridge case wall also increases, and unless this is counteracted by proper lubrication, reliability suffers.
If Marushin had utilized an 5mm cap instead of the 7mm, the case at the junction point would be thicker and the seal at the cartridge base would be wider. However, this might lead to an increase in the production cost of the cartridges due to the smaller piston and O-ring ASSY but Marushin has made considerably smaller and more intricate parts before with no problem.
In fact, the choice of the 7mm cap over the 5mm cap is interesting, due to the fact that Marushin sells both types of the caps, and the caps have the same powder amount.( 0.01 grams of red phosphorus.) and, they produce the same amount of gas pressure.
A possible solution to the weak threaded area might be to solder in a brass tube that has an 5mm internal i.d. and an 7mm o.d. into the cartridge body. Of course, a forward ledge must be provided for a reduced DIA piston (the piston out of the MGC MAC 11 HW should do) so it doesn't fly out of the front of the cartridge! This would increase the cartridge strength and sealing potential.
As with any style of modelgun cartridge (with the exception of the paper cap style) proper lubrication is a must, as the friction of the cap, piston and the O-ring (s) all add up reducing performance.
What is a good idea is to polish the insides of the cartridge walls (actually, it might also be a good idea to polish the exterior also) as this will help to reduce the friction in feeding, extraction and ejection.
This cartridge works quite well if proper care is taken in loading and assembly by using the right grease and paying attention to the reduction of frictional losses (both the cartridge and the modelgun) the firing simulation will be very enjoyable.
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