If I remember correctly, the US Military uses a 1/2 inch pine board to determine if a projectile is lethal. If the projectile can penetrate, it can be lethal to humans. Granted, a .556 generating 1100 foot pounds of energy is capable of much more damage than my lowly .22 Marauder, but both will penetrate a 1/2 inch pine board at 30 yards. The pine board will probably stop the Marauder at 50 yards, where the .556 will continue on, but up to 30 or 40 yards a 20-ish foot pound gun can still be lethal.
No, do not underestimate the power of these little rifles.