I'm having the same type of damage on my 16" M&P10. Went to a Faxon 16" bbl(same AR10 carbine gas), JP SCSH2(heaviest setting/3 tungsten weights and also tried different springs) and Superlative AGB, BA BCG w/ FailZero bolt, and still have those marks. Switching to a Lancer eBCG helped a...