So I took the M4 to the range again today. Instead of shooting all low recoil or managed recoil rounds like I did the first time, I brought some regular velocity (Federal LE Hydra Shok) slugs. With regular velocity, I didn't notice a significant difference between what I shot today and the previous managed/low recoil stuff. 65 slugs shot this afternoon, not even a sore shoulder today (yay!) - nada. May be time to move up to 3" slugs next week...haha...
But the better news is that these rounds fed and ejected perfectly. Not a single problem whatsoever. So I think those who mentioned the break-in period and using regular/high velocity rounds were spot on.
Thing runs like a champ. Never thought I'd look forward more to shooting slugs than my rifle and PCC rounds. So fun!