As part of ongoing efforts to explore the surprisingly fertile intersection between the fields of "scientific inquiry" and "shooting things", I decided to see what happens when .223 ammunition designed for expansion is fired into interior walls.
Inspiration came from the Box 'o Truth website where the author compared penetration of various rounds through simulated walls. Surprisingly, the least penetrative centerfire round turned out to be 55-grain .223--fully jacketed, no less. In some cases, these bullets passed through fewer walls than the canonical home-defense load of 00 Buckshot, thereby simultaneously blowing holes in both drywall and conventional wisdom.
In retrospect, it's not too surprising. Fast bullets are good at penetrating relatively thin rigid surfaces, such as metal, because they punch clean through before the metal has a chance to deform. But when passing through solid objects, speed becomes something of a liability. Viscous drag exerts a slowing force on an object proportional to its surface area times the square of its velocity, but that force translates into a reduction in speed in a manner directly proportional to the object's mass. These two factors combine mean that light and fast projectiles dump velocity much more readily than slow, heavy slugs.
Because surface area and coefficient of friction are factors in a bullet's propensity to slow down when passing through things, it made me wonder: What would happen to expanding .223 bullets if they hit drywall? Theoretically, expanding should increase surface area and therefore reduce penetration even further. But would it actually make enough of a difference to notice?
Referring to the Venn diagram above suggests that answering these questions will require hearing protection.
In the interests of ensuring domestic harmony and minimizing police involvment, simulated walls were used instead of the interior of my house. I built several wall sections simulating the space between studs to test the worst-case scenario of a straight-on wall shot that misses the heavier supporting 2x4s. Building standards call for studs to be separated by 16 inches center-to-center, but these sections are 16 inches outside-to-outside to help them hold together. Hopefully this is close enough to the real thing to realistically simulate the stiffness of inter-stud drywall. 5/8 inch drywall was used in order to comply with all relevant building codes.
The first wall was placed five feet from the firing line and subsequent walls were spaced ten feet apart, which should roughly simulate indoor distances. Separation between the walls is important if rounds flatten out or break up, since this gives air resistance a chance to work against suddenly non-aerodynamic shapes. Spacing the walls so close together and shooting straight on provides a realistic yet worst-case scenario for interior wall penetration.
Rifle ammo was fired from a Mini-14 Tactical, whose 16.25-inch 1:9 twist barrel should produce ballistics representative of most common .223 semi-automatic carbines.
The shotgun used was the canonical Remington 870, specifically a Marine Magnum featuring an 18 inch barrel with a cylinder choke.
Speaking of canonical, a 1911 provided the platform for .45 ACP. It's a custom build belonging to, and built by, the photographer.
9mm ammo was put through an FM Detective, a Hi-Power variant with a shortened slide. Its barrel is about 3.6 inches long, which makes it comparable to most concealed-carry-oriented 9mm handguns. As it turned out, barrel length with handguns probably wouldn't make much difference.
Finally, a Sig P230 provided the platform for testing .380 ACP.