General wound ballistic concepts

There are three concepts generally held by most as to the effect of a bullet striking a human being. The first is that the bullet 'drills' its way through leaving a small entry and an equally small exit hole. The second is that the bullet leaves a small entry hole and an enormous exit hole. The third is that when someone is shot by anything other than an air rifle, the impact is enough to lift the person off his feet and send him flying through the air. Basically, all three concepts are incorrect in one way or another.

Firstly, as a bullet passes through human tissue, it imparts some or all of its kinetic energy to the surrounding tissue (Figure 3.3). The energy so supplied

Temporary cavity

I Permanent cavity

Temporary cavity

I Permanent cavity

Temporary Cavity

5 10 15 20 25 30 35 10 15 50 Penetration

5 10 15 20 25 30 35 10 15 50 Penetration

Figure 3.3 Diagram showing temporary and permanent cavities.

throws the tissue away from the bullet's path in a radial manner leaving a temporary wound cavity much larger than the diameter of the bullet. The temporary nature of this cavity results from the natural elasticity of animal tissue (French, 1962), which allows it to regain its original structure after the bullet has passed. There is also a permanent cavity which results from the destruction of tissue caused by the bullet itself. This permanent cavity is dependent on the cross-sectional area of the bullet and any secondary missiles which may be produced from the break up of the bullet during its passage.

The temporary cavity has a very short life span and is followed by a number of aftershocks decreasing in severity. The final, permanent cavity may be many times greater than the diameter of the missile, but it is also many times smaller than the temporary cavity. Unlike the temporary cavity, where the tissue is merely being thrown away from the wound track and no permanent damage is being caused to the tissue, the permanent cavity is caused by the actual destruction of the tissue by the passage of the bullet.

The dimensions of this temporary cavity are dependent upon the shape, weight, size and velocity of the missile and the elasticity of surrounding structures.

With extremely high-velocity missiles, in excess of 3000 ft/s, there is an explosive movement of the tissue away from the wound track. This results in enormous temporary cavities as well as extensive fracturing to bones and damage to veins and arteries in the immediate vicinity. In addition, there is often a back splash of tissue out of the entry hole giving the impression of an exit wound.

The second misconception, that the entry hole is always small and the exit hole large, is a major factor when interpretation of close range or suicide wounds is called for.

When dealing with high-power handgun ammunition firing hollow-point ammunition, it is often the case that the entry hole is smaller than the exit hole. Tissue entering the hollow- point cavity causes the bullet to expand; in some cases, this can increase the surface contact area of the bullet by up to 200%. Not only does this increased surface area enable the missile to transmit more of its energy to the target, but it also increases the possibility of the bullet damaging a vital organ or blood vessel due to the much larger permanent cavity of the wound track. This expansion of the hollow- point bullet within the tissue will, if the bullet does exit from the body, give rise to an exit hole considerably larger than the entry hole.

Expansion of a hollow-point bullet in soft tissue does not appear to be dependent upon its calibre. It does, however, generally require a velocity in excess of 900 ft/s for this to happen. At velocities below this, the bullet will not, unless it hits bone, expand at all.

Another factor influencing the expansion of hollow-point bullets is bullet yaw. A yawing bullet will not strike the target at 90° and as a result of this, bullet expansion will not occur (Knudsen and Soensen, 1994).

There is, however, some controversy over the necessity for a hollow- point bullet to strike at 90 ° for expansion to take place. Poole etal. (1994) are of the opinion that up to striking angles of 45 °, this is not the case.

Plugging of the hollow point by fabric from the bullet ' s passage through clothing or any other intermediate material will also inhibit the expansion of hollow-point bullets.

If, on the other hand, even a moderately powered handgun is held with the muzzle in tight contact with the skin, the entry hole can be massive. In this case, the high-pressure gases which follow the bullet out of the barrel have nowhere to go other than into the wound behind the bullet. These gases expand at a rate greater than the speed at which the bullet is passing through the tissue and, as they have nowhere else to go, burst back out through the bullet entry hole.

The resultant hole can be enormous and to the inexperienced can give every indication of an exit wound. The presence of partially burnt propellant in the wound and blood and tissue in and on the barrel of the weapon will correctly identify the wound as an entry rather than an exit wound. Another identifier is the deep cruciform tearing around the wound called 'stellate tearing' (Figure

Other indicators of a contact wound can be the flare from the side of a revolver's cylinder and sometimes the presence of a mark made by the front sight of the weapon.

In double-barrelled shotguns, the second unfired barrel can often leave a large impact type mark. This mark results from the high-pressure gases which, before bursting back out, balloon out the tissues crushing the skin into the other muzzle (Figure 3.6).

The third concept, that when someone is shot by anything other than an air rifle the impact is enough to lift the person off his feet and send him flying

Stellate tearing

Barr

Stellate tearing

Barr

Wound Ballistics

High-pressure gases

Tissue

Figure 3.4 Diagram showing effects of contact wound to the head.

Bore

High-pressure gases

Tissue

Figure 3.4 Diagram showing effects of contact wound to the head.

Carbine Exit Wound
Figure 3.5 (a) Stellate tearing, 0.38" Special, entry hole right side of head; (b) stellate, tearing, 0.38" Special, exit hole left side of head.
9mm Wound Ballistics
Figure 3.6 Contact wound from over and under 12-bore shotgun, showing imprint from unfired barrel, foresight and foresight protector wings.

through the air, is completely untrue. This once again enters the realms of mathematical ballistics, but it is a very important concept to be aware of especially when dealing with multiple-shot suicides (Eisele, Reay and Cook, 1981; Stone, 1986).

The common misconception is that after the first shot, the body will be thrown away with such force that a second shot would not be possible. This is, of course, completely false as a brief examination of the mathematics will show.

There are two factors to consider when dealing with the effect of a bullet on the human body; one is momentum and the other is kinetic energy.

Momentum is mass x velocity, and possibly its most important property is that it is conserved during collisions, that is, if two or more objects collide, the total of their momenta is the same after collision as it was before.

If we consider a rifle bullet of 0.02 lb (140 gr) being fired at 3000 ft/s into a 200 lb stationary log, the momentum of the bullet, M1, before collision, is thus

If M2 is the momentum of the log plus bullet after being struck by the bullet, then M1 = M2.

The log will, after being struck, have a mass of 200 + 0.02 = 200.02. Thus, 200.02 x velocity of the log must equal the momentum of the bullet before striking the log, that is, 60 lb ft/s. By rearranging the equation and substituting the known figures, we have:

Velocity of log after being struck = 60/200.02 = 0.2999ft/s

With such a minimal velocity, it can be easily appreciated that if the log were a person, then the body would not fly very fast or far through the air.

The next question is how far would the body be lifted off the ground by such an impact? If the mass is 200.02 and the velocity 0.2999 ft/s,

where

KE = kinetic energy; M = mass of the projectile; V = velocity of the projectile;

200.02 x 0.2992

2 x 32.174

A kinetic energy of 0.27 ft lb is enough to lift 1 l b by 0.27 ft or 200 l b by 0.00135 ft or 0.016 in. A distance of 0.016 in. is hardly on a par with the com-

Table 3.13 A comparison of the velocity, monentum and kinetic energy of various common objects.

Object

Mass (lb)

Velocity

Momentum

KE (ft lb)

(ft/s)

(lb ft/s)

0.22" LR, (40 gr)

0.0057

1200

6.85

120

0.38" Special, (l58 gr)

0.0226

850

19.2

253

Cricket ball (60 mph)

0.328

88

30

40

0.303" Rifle

0.025

2440

61

2 313

12B Shotgun

0.078

1300

101

2 051

600 Nitro Express rifle,

(900 gr)

0.129

1950

251

7 600

150 lb Runner (15 mph)

150

22

3 300

1 128

Elephant (5 t, 20 mph)

11200

29.33

328 500

149700

1 t car (1 mph)

2 240

1.467

328

74.89

KE, kinetic energy.

KE, kinetic energy.

monly perceived notion of a body being lifted off the floor and flung against a wall or through a window.

Table 3.13 comparing the velocity, momentum and kinetic energy for various common objects will place these figures into perspective.

It should be remembered, when using this table for comparison purposes, that momentum is the ability to move the target or to stop it from moving in the opposite direction, and the KE is the ability to lift a weight off the floor.

It is an interesting comparison that the most powerful elephant rifle ever commercially produced would have little or no stopping effect on a charging elephant running at 20 mph.

When talking about the wounding capabilities of a bullet, many people wrongly refer to kinetic energy as 'power'. Power is the rate of doing work and is generally measured in 'horse power'. If the power of a bullet is required, one has to know how long the bullet takes to stop in inches. The kinetic energy of the bullet is then divided by this distance, and then this figure is divided again by 550 to convert the answer into horse power. Thus, the power of the bullet is dependent upon the material into which it is fired. As a result, this is a fairly meaningless figure.

Generally speaking, the only way a body is likely to leave the floor after being shot is because of involuntary muscle spasm caused by a shot to the brain.

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Responses

  • Michael
    What is the ballistic concept?
    8 years ago
  • Patrick
    Is the entry wound larger than the exit wound from a high power rifle?
    8 years ago
  • Eric
    What are the basic misconceptions about general wound ballistics?
    5 years ago
  • Daryl
    Does stellate tearing happen in suicides?
    3 years ago
  • Mark
    What are the general wound ballistic concepts?
    5 months ago
  • ADIAM
    What is the wound ballistics of being shot in the head called?
    2 months ago

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