I recently made a post on the Bear's Pit Forums for Jagged Alliance 2 with regards to a bullet-damage balancing thread, where people were discussing the wounding effects of bullets and how best to simulate them.

Everyone knows that a bullet can kill. Everyone also knows that the placement of a bullet matters a great deal; a shot to the head is almost always instantly fatal, a shot to the aortal bridge results in clinical death by exsanguination within twenty or thirty seconds and incapacitation nearly instantly, etc.

However, when you ignore the variations in shot placement -- or assume that you have averaged the kill shots with the wounding shots -- how do you calculate, dead reckoning, how much damage a bullet inflicts against a given target?

Here is my interpretation:

Assumption: If the bullet cannot penetrate skin or armour, the bullet causes a bruising wound proportional to the impact velocity, reduced by the armour's ability to diffuse impact damage. Hard armour diffuses damage worse than soft armour does.

Assumption: If the bullet penetrates skin, the blood can flow out through a circle equal in size to the entry wound. Though entry wounds can be anything from ellipses to simple tears in the skin, calculations are much simpler if they are always a circle proportional to the bullet's calibre.

Assumption: A bullet's calibre causes the bullet to create a hollow cylinder as it passes through the body. The tissue isn't actually "cut out" of the cylinder -- the tissue is simply sliced open on the point of the bullet and displaced. As such, the wounding effect of the bullet can be assumed to be the surface area of the cylinder: how much flesh is touched at one time by the bullet.

Assumption: If the bullet stops within the body, the cylinder stops at that point. Thus, the kinetic energy and penetration depth of the bullet is also a significant wounding factor. A low-energy .38 special bullet will usually stop within the body, so its cylinder won't be comparable in length to a high-energy .22 bullet.

Assumption: The body is assumed to be 30 centimetres (one foot) thick.

Assumption: A bullet expends "x" amount of kinetic energy to penetrate the skin. It expends "y" amount of kinetic energy per centimetre (roughly 17/32'') of flesh passed through. After passing through the flesh, it must expend another "x" amount of kinetic energy to penetrate the skin on the far side. If it does not have sufficient kinetic energy, it stops inside the skin.

Assumption: A bullet which tumbles or fragments causes severe wounding properties and ragged wounds. Bullets which tumble or fragment multiply the damage suffered per centimetre. However, fragmenting bullets lose energy faster, which translates to a reduced penetration depth -- tumbling bullets also lose energy, but not nearly as fast.

Assumption: A bullet that penetrates the skin on the far side creates an exit wound the size of the entry wound. Physically-speaking this just isn't true, what with Desert Eagle Action Expresses leaving exit wounds the size of basketball courts and all, but vastly simplifies things.

Final Computation:

The damage of a bullet is equal to:

1) If bullet does not penetrate, no lethal damage. Stun damage proportional to all of the bullet energy.
2) If bullet penetrates first layer of skin, the bullet causes n*1 bleeding, where n is equal to the area of a circle with diameter equal to the bullet calibre. Subtract bullet energy equal to the amount of energy needed to penetrate the skin proportional to the bullet calibre.
3) For every centimetre of flesh the bullet passes through, add damage equal to the surface area of a cylinder 1 centimetre in length (pi*calibre(cm)). Multiply damage by "tumbling factor" and "fragmentation factor". Subtract bullet energy per centimetre proportional to an area of a circle equal to the bullet's calibre as well as geometrically proportional to the "fragmentation factor" squared and directly proportional to the "tumbling factor".
4) The bullet strikes the layer of skin on the opposite side of the trunk. Subtract bullet energy equal to the amount of energy needed to penetrate the skin proportional to the bullet calibre. If the result is greater than zero, the bullet creates an exit wound; the bleeding damage is set to n*2, where n is equal to the area of a circle of the bullet's calibre.
6) The target suffers stun damage equal to the percentage of bullet energy that is actually transferred. Any remaining energy which passes through the person is lost.

How does that sound?