https://en.m.wikipedia.org/wiki/5.56×45mm_NATO#

“There has been much debate of the allegedly poor performance of the bullet on target in regard to stopping power, lethality, and range. Some of this criticism has been used to advocate an intermediate-sized cartridge between the 5.56 and 7.62 NATO sizes.”

The AR15 is typically chambered in .556, a medium sized cartridge typically pretty lacking in stopping power especially when compared to other popular firearms like the M1 Garand chambered in 30-06 much actually does penetrate much further that .556