This is just some of the interesting and informative information from this website: http://www.ammo-oracle.com/
Q. Isn't 5.56 too dangerous to use indoors? Shouldn't I use a pistol or shotgun instead?
Virtually any kind of ammo, with the exception of light bird shot, will easily penetrate typical wall construction (two layers of wall-board separated by 3 to 4 inches of space). Testing has shown, however, that after penetrating a typical interior wall, a 5.56mm projectile will have less wounding potential than most common handgun or buckshot loads. This is true because the low mass of the bullet sheds velocity quickly, and velocity is its key wounding component. This doesn't mean that 5.56mm ammo isn't still potentially deadly, but that the severity of an injury is likely to be less from a 5.56mm bullet than from a 9mm, .40, .45, or #00 buckshot round. What is important is not the degree to which these rounds penetrate, but their "ex post lethality" or their lethality AFTER encountering wallboard or other cover/concealment.
The difference is so significant that the FBI and other ballistic experts recommend that law enforcement transition to handguns to "dig suspects out" of cover because of the superior penetration and wounding ability of handgun rounds over 5.56 or .223.
This, along with the increasing number of lawsuits from "friendly fire" submachine gun victims and 5.56mm's ability to penetrate ballistic vests, are some of the reasons that many SWAT teams are transitioning away from the 9mm MP5 and selecting 5.56mm carbines instead.
This is understandable given the longer barrel length and therefore higher velocity and consequently higher penetration of handgun rounds in submachine guns.
If our experience on the forums are accurate, most shot gunners and submachine gun fans receive this news poorly. It does seem counterintuitive since 5.56mm is a "high powered round." All we can say to this is that the FBI FTU fired hundreds of rounds through carefully constructed wall sections and then into gel. Ignore these results at your own peril.