I've had eric_james' MP-40 apart and it definately uses a bellow systems. PGC makes a "normal" gearbox conversion that's pretty pricey, like about $300 usd and does not include cylinder, piston, gearset or motor. Eric want to get one and have me install it, but it'll cost more then the gun
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Nice gun, hard to get mags for though. There's a hicap available now if anyone has them in stock.