foam in the stock - not the expanding type, it will break it. Or bits of rubber matt (and foam). old rags. you get the idea.

Firing cycle - biggest gain is to find a spring that fits the guide properly, to stop the twang. Then add a piston sleeve to stop it rattling around inside the piston, and gob some grease on it all - you are not going for FT level consistency.

All that's then left will be a bit of muzzle crack - that's part of the fun, so leave it alsone... but obviously a silencer can be fitted if you really must.

PS Don't fit a PTFE piston head