From your above sounds like it could be the mags feed lips.
Load a full mag. Pull charging handle all the way to the rear and let it go. Take out mag. Pull charging handle all the way to the rear again ejecting ammo. Does the brass have two distinct score lines on it running the entire length of the bigger part of the brass ??
If so, it is the mag feed lips digging into the brass,slowing down the fwd speed of the BCG. It might not do this with just one or two rds in the mag because of limited upward pressure from the mag spring.
Smooth/contour the underside of the mags feed lips. That should take care of the problem.
Guns chambered in 308Win,223/5.56,and others will do the same thing depending on which mag you use. This is why I do not use anything but the MagPul brand in all my guns.
Let us know what the "fix" is. --- SAWMAN