your probably going to need a large pressure vessel, if its anything like my mac-11, which you can see the gas cloud expand out about 4 inches from the muzzle during full auto fire. Having such short barrels, the gas will not see nearly as much expansion before leaving the muzzle as it does in a long-arm, which is why it can take a larger amount of gas to get a higher fps in shorter barrels, so there is a larger amount to compensate for with a smaller barrel. I would imagine you would probably need a pressure vessel about 2 inches in diameter and at least a foot long to contain all of the gas, this is very rough guesstimate though, it would probably take a bit of experimentation to get right, and I'm fairly certain I got mine right by magic :Y
Feel free to message me if you need assistance with a WE-scar, gbb maintenance, or any maintenance for that mater, I'm around and checking.
WE SCAR, "King's special" (in for repairs)
Cyma AK-47 - "bloodbath"
KWA Mac-11, for making things loud in a hurry
Tanaka m700 breakdown, for those rare situations when I don't actually want to be seen