I had what I believe was the same issue happen the other day toward the end of a practice session, never happened before. The last couple mags (33 rounders) I shot, had several rounds feel like puffers and cycled slow, mostly in the last 40 rounds. I had not cleaned or oiled the gun in several hundred rounds. The gun is a GMR 15 with approx. 2,000 rounds down the tube. I cleaned and oiled the gun and went back out today and shot 220 rounds through the same mags , used the same ammo and even mixed in some factory. Did not have the issue on a single round.
I just thought I would throw in my experience.
Thanks,
SR