Originally Posted by
mileageman
ATRS had the spacing wrong of the FA in relation the the bore the BCG travels in on the first batch FA uppers they made, it was too close to the bore by about 0.04" if I remember correctly, so the FA might stick into the bore the BCG rides in and rub the Bolt Carrier. Some FA assist fit no problem, some didn't, depending on the BCG you had. ATRS as far as I know have fixed this issue and going forward this shouldn't be a problem with any FA upper, as long as it wasn't from the first batch they made.