True, but the piston's location isn't usually in the chamber area as is the gas port on the AR platform. In the Garand it is far forward near the muzzle and in the M-1 Carbine it is below and forward of the chamber, isolated. Allowing gas exhaust and particles in the chamber area (behind the throat) to me is a bad idea.....
No the piston is bolted to the barrel which is what makes a piston rifle innacurate. Do you have a piston rifle? Have you ever broken an oprod?