UNDERSTANDING BULLET PENETRATION

This section will be devoted to discussing those factors which influence bullet penetration through animals and the importance of bullet spin rate (RPM). The discussion applies only to expanding bullets.

In an animal, there are different tissues which are of differing densities and hardness. The frontal skull bone is typically the densest and hardest, and causes the most deformation and fracturing of bullets. The next hardest is long bone since it has to be weight bearing, then comes the spine then the pelvic bone, ribs and shoulder blade. The least dense bones are the sinus bones which are very thin. As a general rule the bones will deform expanding bullets the most and are most likely to cause tumbling. Next is soft tissues like muscle, intestines, solid organs such as the liver, kidneys, spleen and heart. The least dense are the lungs which are quite heterogeneous in that they contain vessels to maximize blood flow at low pressure. Finally the hide, or skin. It can vary significantly in it will be the first tissue the bullet will encounter on impact.

Given this anatomic heterogeneity, one should immediately realize that ballistic gels are only a poor first approximation of animal soft tissue and are very homogeneous in density. Given this reality, gels are easier on bullets than animal soft tissue, and as such offer much more uniform resistance to a bullet as it passes through. They are useful in seeing how an expanding bullet expands, so the designer can see what parameters affect the expansion of the bullet under consideration.

The material the bullet is made out of profoundly affects its penetration and expansion. Bullet materials for hunting are usually all lead, copper jacketed lead core, and “all” copper, or brass, a copper alloy. Lead is very “soft” (high ductility) but also very dense and relatively inexpensive. Copper is less dense, considerably less ductile than lead and therefore harder and more brittle than lead. It is also more expensive. Cost aside, both metals are used extensively in bullet making. The high ductility of lead, caused it to resst pressure and stress poorly. Thus, lead and lead core bullets are subject to more deformation and even splattering on impact with soft tissue at high impact velocities.

Before we get into how the material characteristics affecting penetration through soft tissue, lets consider the basic physics involved first. Consider the case of 2 bullets of the same weight, one having a meplat of say 0.050″ in diameter vs the second having a meplat of 1″ in diamenter made out of the same material of the same hardness and ductility. Assume both are traveling at exactly the same speed when they impact a soft target. Both will have the same kinetic energy (ft-lbs) but the pressure at the point of impact will be vastly different in terms of pounds/ sq inch of impact pressure. The area, A, of a circle is described by the simple equation, A=(1/2 diameter) squared x 3.1416. Say both projectiles have the same ft-lbs of energy, the pressure at the point of impact will be determined by the difference in diameters of the meplats. Since the smaller meplat projectile has a meplat surface area 20,000 times as small as the larger meplat, applying the above formula to calculating the difference in pressure (lbs/inch square) seen at impact indicates the pressure at the 0.050″ diameter meplat will be 400,000,000 times that of the 1″ meplat projectile. Because the area of impact will vary by the squared function of the frontal surface of the bullet, small differences of the frontal diameter of the penetrating bullet will lead to large differences in penetrating pressure. Because lead core bullets are much much softer than copper and expand to larger diameters, they rapidly disperse their penetration pressure over that larger surface area and will not penetrate nearly as well. At high impact speeds, lead core bullets with high BCs thin jackets propelled faster can hit with enough force to fully overcome the strength of the lead core and jacket, and splatter like a spit wad on a wall and not penetrate at all. Increasing speed and BC of a lead core bullet can be very detrimental to its penetrating abilities at close range. Not so with all copper bullets.

Finally, petaling copper bullets are able to use the rotational energy imparted to the bullet by the rifling much more efficiently than expanding lead core bullets. To have some idea of how fast a “typical rifle bullet” is rotating consider a bullet regardless of caliber, with a muzzle velocity (MV) of 3000 fps exiting a 1:10 twist barrel. Ignoring the force of friction, bullet is making 1.2 revolutions for every foot it travels. In 1 second it will travel 3000 feet X1.2 or do 3600 revolutions/second X60 second/ minute means the RPMs are 216,000 RPMs. By the same calculation that bullet launched from a 1:7 twist barrel at the same speed will be spinning at 308,571 RPMs. Lead core bullets expand to creat a smooth frontal surface which doesn’t do much except slow the bullet rapidly. The petals on an expanded copper bullet are sharp, have rough surfaces and are separated from each other. As the bullet moves through the tissue, the tissue is forced in between the sharp petals then liquified by the high spin of the bullet, like a propeller in water. That liquefaction is fluid and offers less resistance to the copper bullet, so it will penetrate a lot more deeply. The typical expansion diameter we have designed into our bullets is about 1.5-1.75 calibes. Copper bullets “designed” to shed their petals immediately, take less advantage of the rotational energy of the bullet, lose mass quickly and rely on the weight of the remaining shank for penetration. Nevertheless, because of preservation of the shank, they also tend to penetrate better than lead core bullets going the same speed.

Penetration for hunting bullets is extremely important, allowing more paths of varying length to reach and disrupt the vital organs and effect a rapid and virtually painless death. Quartering shots at close range with high speed high BC lead core bullets are plagued with insufficient penetration causing unreliable rapid kills. Taking of mature animals with reliably expanding copper bullets who have procreated for several years prevents their dying from starvation, disease, freezing or being eaten half alive by predators.

The result is that all copper expanding bullets of high BC make the most highly penetrating and destructive hunting bullets available. The higher ductility, machinability and high enough density of copper makes for a bullet that is capable of withstanding very high rotational speeds, and high impact velocities with the widest performance envelope possible, much wider than lead core jacketed bullets.