I just saw a episode of Midway's "Shooting Gallery" where he used- IIRC- a 55 grain bullet in an AR platform to show that- contrary to "myths" about over-penetration- that the 55 grain (5.56) round actually has less than many popular handgun rounds due to the fact that it becomes extremely unstable upon contact with a barrier. Can't recall whose bullet it was, though.
Tests were done at Gunsite with ballistics gel, but there was no hard data provided. Set your Tivo to record the series, it'll probably be back on again soon.
07 FFL /Mosin-Nagant Custom Shop/Bent Bolts
Gun-Kote/ Cerakote Refinishing
Criterion Barrels/ Savage Re-Barreling