There are four basic ways that a bullet can kill a game animal or an enemy combatant.
(1) The largest game animals are often shot with a “solid” bullet, which is designed to cut a straight path through the animal. It does not fragment, or yaw (tumble), or mushroom to any significant extent. This approach is chosen of necessity, because of the thick hide and heavy bones of these animals.
However, the approach is far from ideal for enemy combatants. When military bullets fail to fragment or yaw or mushroom, they are much less effective at killing.
(2) Fragmentation is used in modern military ammunition, and it works well as long as the bullet velocity is sufficient to cause fragmentation and as long as there remains some sizeable portion of the bullet to penetrate further. However, at close ranges, too high a velocity can cause the bullet to fragment at a shallow depth, leaving nothing to penetrate deeply into the vital organs. Then again, at extended ranges, the bullet velocity is necessarily lower, and there may be no fragmentation at all.
This approach to military ammunition is problematic because it requires a narrow range of velocities, not so fast as to fly apart, and not so slow as to fail to fragment. Hunters use fragmenting bullets only for varmints — small animals where deep penetration is not required for an effective kill.
(3) Bullet yaw refers to a turning of the bullet nose away from the direction of travel; it is a tumbling of the bullet. But in military use, the bullet does not tumble again and again; it makes less than one revolution. So the term “yaw” is more accurate than “tumble”.
The same problem that plagues fragmenting bullets also affects this approach. At closer ranges, the bullet velocity may be too high for yaw to occur. In ballistic gel, the yawing may occur at a depth greater than the depth of the military target (the enemy combatant). At longer ranges, the bullet velocity is too low to initiate yawing at all. Hungers do not rely on yaw for killing game animals; it is not reliable.
(4) The type of bullets most favored by hunters are those that expand (“mushroom”) and penetrate. For this effect, the front of the bullet has a thin jacket and typically a lead alloy core. In order to effect expansion at lower velocities, often a hard plastic tip is used. The base of the bullet needs to resist expansion, so that after expanding energetically, the base can penetrate the game animal’s vital organs.
This design, properly implemented, allows for an effective kill at close range and high velocity, as well as at longer ranges and lower velocities. Perhaps the best example of this approach is found in the AccuBond-LR bullets, which have an optimum velocity range from 3200 fps all the way down to 1300 fps.
Although the AccuBond-LR is marketed as a hunting bullet, it has all the features needed in a military bullet. If the enemy is close, the bullet must kill effectively at high velocities. And if the enemy is further away, the bullet must still expand and penetrate effectively. At all useful ranges, the bullet must both mushroom and penetrate.
Hunters have long known that the most effective bullets for medium sized game are those that mushroom, rather than fragment. The military would do well to abandon the approach of bullets that supposedly tumble or fragment, and use medium weight bullets (120 to 150 grains) that expand and penetrate, over a wide range of velocities.