A 5.56 fmj will tend to pitch and yaw some after entry. I think it starts to do that after about 4"-6" depending on the load, distance, etc...but the term "tumble" is a terrible description iof what the round does.
All bullets pitch and yaw after impact. If they have significant levels of it in flight something's terribly wrong with the firearm. AR's are no different. My 600 yard targets always have clean cookie butter holes