Paul Howe has specifically mentioned M855 as yet another reason why shot placement is always King. If you haven't seen it yet, here's a DocGKR 2008 presentation that touches on some of the variables.
ATK used to (or still does?) hold "Wound Ballistic Workshops" at hosting agencies where they'd set up gel and barriers to demonstrate their newer ammunition vs. host agency's current issue ammo. Not the best controlled environment due to field conditions of varying gel temp, barrier angles not exact, etc. What appealed to me was the cumulative consistency of ammo performance over those varying conditions. They had some WBW info on their website but a lot more available if you asked.
As DocGKR says... we have good ammo today and terminal ballistics vary from shot to shot. Individual shooter performance is the real deciding factor and that's where our time needs to be spent.