Just put a 6 position stock on my 20" M&P AR15 in 5.56.What buffer would work best for all bullet weights between 40 and 69 grains
Thanks.......... Buckey

I use a standard carbine buffer in both my mid-length and carbine length gas system guns. I value 100% function over all else.

You probably have rifle length gas system on a 20" gun. Standard buffers should work fine. The heavy buffers are usually used with over-gased carbines.

on my carbine i shoot 55gr and ive run 4,000 rounds without one hiccup. if it aint broke dont fix it. on semi auto you shouldnt need to change anyway maybe on full auto to make recoil more stable.