Nothing is really "wrong" with it. The reciprocating barrel system is actually like a reversed closed bolt operation - pretty cool either way. Just that the timing in the gun had to be perfect and if it wasn't, the barrel would reciprocate at the wrong moment in the cycle thus chopping the ball.
Other problems with it include the lack of the 2 stage bolt system. Without this, the bolt remains at a preset pressure - around 90psi if I remember, but that's just for the bolt pressure. More pressure is used to make the barrel reciprocate and the problem is that it isn't adjustable.
Aside from that, some of the earliest versions of the 80 had problems with the boards and trigger. Others had misaligned feednecks as well.
Hope that helps.
The Original WarMachine Factory Team
SN's a nationality - not a shoe shine...