To elaborate:
The M16/M4 platform is, unlike most calibers, suffers dramatic changes in performance when assorted factors vary. Barrel length is significant, as is twist rate, bullet weight, expansion/fragmentation behavior, velocity, etc. Seems it works quite well when all these factors are tuned correctly as required for certain needs ... but use the wrong parameters under certain conditions, and of course it doesn’t work as desired.
Biggest problem I can tell is the insistence on using “green tip” rounds on soft targets using short barrels. Use Mk262 and it works fine; use a lighter round that won’t yaw/fragment, well duh performance is sub-optimal.
The insistence on 1:7 twist, combined with lowered velocity and 62gr steel core pill, leads to subpar terminal performance.
Given M855 and 1:7 twist, the short barrel is a poor choice.
Additionally, the short gas system reduces available power to cycle the action compared to mid or full length gas systems.