• Ken Kraushaar

notes on overpenetration

good morning all,

hope this finds you well.

I wanted to write some notes about this topic because it is garnering a lot of attention in terms of the raging debate about intermediate round rifles and their effectiveness for CQB or home defense.

as is usually the case, armchair opiners with little firearms knowledge are once again making the case for why you shouldn't have an ar-15 and how it is a poor choice for home defense.

they site that because it is a "high velocity" round, that it therefore must penetrate more than pistol rounds, and therefore, only pistol rounds would be viable for home defense because you wouldn't want to accidentally hit little johnny down the street after the bullet penetrates the walls of your home.

due to the bullet's weight, and design, the opposite is actually true. a smaller faster round is actually impeded more by things like drywall, insulation, etc, than pistol bullets.

the reason for this is in large part due to the same phenomenon that causes small caliber bullets to potentially tumble when they impact flesh, namely yaw, caused by the hydrodynamic forces involved once a projectile impacts a surface, as it transitions from an aerodynamic interaction to a hydrodynamic one. bullets themselves have about 3 phases of interactions: interior ballistics (i.e. what acts upon the bullet inside the gun), external ballistics (i.e. what it does in flight), and terminal ballistics (i.e. what the bullet does when it impacts something.) yaw encountered when the bullet transitions from an aerodynamic interaction, to a hydrodynamic interaction is encountered in the terminal ballistics phase.

this is touched briefly upon in an article from outdoorhub.com (2013) regarding whether or not the ar-15 is actually good for home defense, in which the author built simulated walls, both hollow and not, and shot both a 9mm bullet, as well as a .223 bullet at them to see their interactions.

independent of the bullet type used, both bullets passed through several walls, when shot at ranges typically encountered in a home defense situation, usually about 5 feet, in this case.

the author noted that the more dense and larger diameter 9mm bullet passed through each successive wall, largely unimpeded, while the .223 began to deflect considerably, and depending on material construction of the bullet, actually fragmented. the reason for this is due to the lack of mass and density of the .223 compared to the 9mm, and the material stresses involved once it the projectile comes into contact with an object.

the less dense, the more the bullet becomes impacted by yaw and hydrodynamic forces. this area is what is encountered in the terminal ballistics phase.

due to these stresses upon the bullet, it is more likely to affect a less dense faster moving bullet than a slower moving more dense bullet, with the more dense bullet being largely unaffected while it passes through the walls, whereas the faster lighter bullet will be impacted more due to the fact that there is less mass to physically move the wall out of its way during flight, and is more likely to be subjected to tumbling, shear stresses and disintegration.

while it is counter intuitive to those without firearms or ballistic knowledge (i.e., those who pay more attention to what the media states about a gun, and not real world hands on knowledge), it is pretty extensively studied by ballisticians , as well the FBI and other law enforcement agencies, specifically when determining the efficacy of employing or deploying a particular caliber or firearm inside, keeping in mind that a large percentage of LE encounters and FBI encounters will take place indoors, and as such they are primarily concerned with overpenetration for this very reason.

anyone wishing to further read up on the physics involved should seek out the books by donald carlucci, as well as vincent di miao, as both cover what happens, particularly when it comes to forensic ballistic investigation and establishing an understanding of terminal ballistics with which LE has to work with.

as always, stay safe and happy shooting.