Slam firing except when on safe????

The safety just prevents the trigger from being pulled and keeps it from pivoting. When the piston is cocked, the 1st sear is pushing down hard on the trigger and any slight play will allow the sear to drop.

Put the old cylinder back in and try again. If the slam fire goes away, it would mean that the new piston may have a smaller diameter allowing the sear to release it prematurely.

Oops, my bad.

I have that exact problem on my M28 and found what caused it and fixed it.

Check your 2nd sear. Betcha the sides look chewed up and it can wiggle or flop side to side.

When you cock the bolt back and the piston engages the 2nd sear, it jams it either to the left of the cylinder slot.

That slot is punched out and has rough jagged edges that grabs the 2nd sear causing the stick. When you push the bolt handle forward the 2nd sear is forced downwards slightly allowing a piston slip giving you a slam.

I used 220 and the 400 grit to polish the slot edges and got rid of the slam.

My cylinder is so much smoother when I recock. Also check the inside of the receiver. casting burrs can cause sticks as well.

Any CNC milled trigger group set will be better than the stock cast pot metal ones.

The trigger pull will also be crisp and feel so much nicer when you pull it.

It depends how much slop is in both sears.

This is slop.