This is on a Polish PMKMS kit from Global Trades.

The darn thing is on there tight and won't budge

Look for a small circle on the tapered part of the bolt carrier, just next to where the piston screws into. It may be a bit hard to see, but this is the pin that holds the piston in. It must be drilled out, similar to how the rivets are drilled and then removed. Only then can you unscrew the piston. To install a new one, screw it in all the way, then unscrew slightly to allow for a small wobble. Then drill the new one to line up with the holes in the carrier. Its easier if you line up the carrier holes on your drill press with the use of a drill press vice, then screw in the piston. Once the new hole has beed drilled , I use a regualr nail to replace the pin that you have just removed.  cut off the head of the nail, and peen the ends with a small hammer. Then use a dremmil to clean it up.

I pound them out with a roll pin punch, put the piston in a vise on an angle so that it rests on the taper of the piston near the threads, crank down the vise and beat away, they come right out. Some pistons are solid where the threads are and some are hollow, i use pistons made by a fella over at akfiles.com by the name of Hotbarrel, his pistons are primo (stainless) and cost fifteen bucks shipped. They are hollow under the threads which makes it easy when drilling, drill through one side, flip it over and drill that side, i use a piece of 1/8 inch solid rod (any hardware store) cut it down to size, install and plug weld over the holes, finish up with a grinder and smooth it out. McM