I've been shooting M16/ARs for over 40 years. Seen several abused, seen them shoot in combat, in my NG Marksmanship unit and in high power and 3 gun matchs to present.
I have never heard of a bolt carrier breaking. Not saying it can't happen, anything is possible.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071