Shooting my Gemtech equiped Bushmaster yesterday for the first time I was using brand new 55 grain FMJ Remington/Federal ammo and after being fired the brass was filthy. I have never seen once-fired brass so covered with black powder residue, the outside of the cartridge case was almost the same color as the inside. Is the extra gas blow-back is causing the issue or is this common for the AR or do I possibly have a problem with this rifle? TIA