When reassembling a model 51,does the buffer piston on the barrel need to be positioned in a certain way?It has a milled slot.Is the position of this slot critical? Thanks rr

Yes, the slot is intended to line up with the retainer spring. The spring then drops down into the slot. the retainer spring looks like a small piece of wire with a small 90 degree bend at the end that would drop down into the piston slot.

Thanks,made a retainer from coathanger wire.Ran 12 rounds through the gun.Worked perfectly.