I installed a Sdalak TIN coated NM piston in my M1A NM,the orginal is still good looking,could this be a reason for my group size?they seemed to grow latley.Should I replace the gas cylinder and piston together when the piston wears out,and how many rnds.before I change? what should I look for with the piston to see when it needs changed.My M1A NM was made around the late 80's,it is all orginal USGI parts,not sure of the rnd.count I bought it used from a friend,and I've proably put 150rnds.through it myself,and I'm pretty sure that he has put less than a 1000rnds.through it before me.