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.
Fact: Interestingly enough, in FBI Firearms Training Unit tests show that submachinegun and handgun rounds penetrated more on average than .223/5.56mm rounds in typical interior construction and tissue.
Opinion: Generally high velocity rifle rounds fragment so readily that over-penetration in an urban (indoor) setting is LESS dangerous than with handgun or submachinegun rounds like 9mm, 10mm, .40S&W, etc. 5.56 FMJ rounds will do more penetrating than JHP and JSP rounds but still are generally safer for interior use- insofar as bystanders are concerned.
Q. Won't JSP and JHP rounds be safer indoors? Don't I have to worry about FMJ rounds going through walls and hurting my family or others?
You always have to worry about it, of course, but even FMJ 5.56 rounds will have less wound potential after penetrating a wall than even 9mm handgun rounds. Generally after passing through an interior wall or two, 5.56 bullets will have lost enough velocity that resulting wound damage would be greatly diminished. It should be noted, however, that all of the above bullets are still potentially deadly to those on the other side of a wall, so plan accordingly. Interior walls are concealment, NOT cover.
Fact: Evidence increasingly shows that 5.56 FMJ rounds like M193 and M855 are not the over-penetration risk they have often been though of as. In interior wall tests, 5.56 rounds have less wounding potential after wall strikes than any common 9mm or above handgun ammunition and/or 00 Buck shotgun loads.
