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Sat-Sun: 9am*-7pm

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Bullet Penetration

One never ending source of discussion and dissension is when different calibers and bullets are compared. Numerous studies have delivered many differing results when it comes to how different types of bullets react when hitting a target. Since the design of bullets are geared towards their impact upon soft tissue for purposes of hunting, self-defense and military the majority of studies have concentrated upon this aspect of terminal ballistics.

A bullet travelling through the air has a couple of basic physical attributes. It has a given mass (in the firearms world this mass is measured in grains) and velocity (usually measured in feet per second [fps]) as well as a rate of spin. The mass of the projectile is constant, the velocity depends upon the type and amount of gunpowder in the bullet case as well as the length and type of the barrel, and then upon the distance from the firearm (the closer to the firearm the faster it is). The spin depends upon the type of rifling in the barrel and also upon the distance from the end of the barrel. All of these factors plus various other environmental factors such as ambient temperature, humidity and air density can affect the bullet and it's kinetic energy remaining when it stops flight through air.

What happens to the projectile upon hitting a surface depends upon the composition of the bullet, the shape and form of the projectile as well as it's rotation rate and speed. In addition, projectiles will react differently depending on what type of target they hit. Some projectiles, notably armor-piercing and military ball rounds, are designed to not break up and deform for as long as possible, thus penetrating as deeply as possible; other rounds such as handgun "self-defense" rounds are designed to deform as quickly as possible and to not over penetrate. Since so many different permutations of conditions (speed, target type, distance, angle, bullet type, etc.) exist there is little generalization that can be done here. Every major manufacturer of ammunition has conducted tests of their rounds as have numerous private and public institutions. The results are not always clear and often even contradictory.

Bullet penetration in homes

A consistent worry with people thinking about the self-defense uses of their purchased firearms is over penetration of bullets within their abode - be it a house or an apartment. The same applies to potential uses outside, either into or from a vehicle or elsewhere. Once again Hollywood has done us a great disservice in either exaggerating or de-emphasizing effectivity of firearms; in some films a small .32ACP round will blast an assailant off his or her feet and have them fly several body lengths through the air from the impact. On the other side of the power curve some films show actors taking cover behind flimsy drywall or plywood and bullets seem to bounce off this surface (usually to the sound effect of ricochets). None of these scenarios come close to reality. Many people also assume that because rifle bullets are designed to travel far and are higher-powered than handgun rounds that they will also go through numerous walls before stopping. This is not accurate.

Self-defense bullets are most often hollow-pointed, meaning that the tip of the bullet is concave and not a solid sharp point. This type of shape assists the bullet "mushrooming" or deforming when it hits a surface, thus causing a larger frontal surface area and subsequently slowing down faster - in the process transferring the kinetic energy to the target. The idea behind this design is to ensure that the bullet will not over penetrate and continue flying on through and past the target. Even though rifle rounds such as the .223 Remington and .308 Winchester usually have pointed tips their design is such that the bullet starts to tumble and break up upon penetrating a surface and therefore do not over penetrate.

Typical construction materials used in Arizona consist of wooden frames and drywall. Studies have shown that both defensive handgun (hollow point) and rifle rounds can travel through 2 or 3 drywalls but then penetrate no further. This would equate to one interior wall and then the exterior wall. The amount of energy and projectile size after going through one wall with 2 layers of drywall is significantly reduced in both types of rounds and may already be considered less than lethal. A recent Mesa Police Department study shows that rifle rounds pose less of an over penetration risk than do handgun rounds; one curious effect is that some hollow point projectiles will hit a drywall and have their cavity fill up with the material and subsequently act as ball-rounds. When walls are made of cinder block or brick the study has shown that rounds will not penetrate and no penetration occurs using defensive handgun and .223 caliber rifles. So the myth of a round going through the house walls then the neigbor's car and finally their TV-set remains exactly that - a myth.

Ball rounds vs. Hollow-points

Ball ammunition (either military or practice ammunition) is designed to keep it's shape and form upon impact, whereas hollow-point or defensive ammunition is meant to deform and expand upon hitting a solid obstacle. Thus, self-defense rounds tend to leave their energy in the object hit while ball ammunition will continue through the object and thus tend to damage less. Please look at our Bullet Expansion article for pictures and detailed descriptions.

Some additional reading on ballistics:
FBI Ballistic Test Protocol
Synopsis of the FBI Ballistic Test Protocol
Bullet Penetration Demonstration Fact Sheet