UMC is garbage ammo. I'd rank Russian ammo above UMC.
On a related note, my 20" AUG would intermittently fail to lock the bolt back after the last round. Using a variety of M193, M855 and Wolf Gold in AUG mags and Pmags it would happen about every other mag change. This went on for a few hundred rounds, then I turned the gas setting up to the adverse setting and fired a few hundred more rounds with no issues. Turned it back down and it's been fine since.
Like someone already mentioned, these guns ship with very strong recoil springs and feel very tight. So use full power 5.56 and maybe even turn up the gas setting to break it in a little faster. I couldn't tell the difference between the two gas settings when shooting, there was no perceived difference in recoil or anything to me.