While some people question the 12" penetration limit, it is not subject to discussion in this article. The FBI is deemed to be more knowledgeable than most, and it is backed up my Dr. Martin Fackler and others who have spent their life discussing the subject. Duncan McPherson, in his book "Bullet Penetration: Modeling the Dynamics and the Incapacitation Resulting from Wound Trauma" actually argues that 15" is not an unrealistic requirement a bullet should obtain. He does point out, however, that 11.5" of penetration shouldn't completely disqualify a bullet from being acceptable either. While 12" should be a minimum requirement, 18" is the approximate maximum desired penetration depth. Beyond that, and the bullet is likely to exit the intended target and retain enough energy to cause others harm if a person should be in the line of fire. Obviously you should never take the shot if you're not sure of what's beyond your target and rely on your ammunition to do your job of being prudent.