In skin and fabrics, it is, unless a wadcutter-type bullet is used, all but impossible to determine the calibre of a missile from its entry or exit hole.
Wadcutter bullets, as discussed earlier, are intended for target practice. As such, they are designed to cut a clean hole through the target to facilitate the determination of the shooter' s accuracy.
When round-nosed or even hollow-point bullets are used, the hole produced by the bullet is very much smaller than its calibre. In skin, this is caused by its natural elasticity, which allows the bullet to force apart the cell structure. After passage of the bullet, the skin regains its original shape exhibiting only a very small entry hole surrounded by a bullet wipe mark much smaller than the original calibre of the bullet.
In fabrics, it is the weave which separates, allowing the bullet to pass. Often, torn fibres will be visible, but these indicate little other than the direction in which the bullet was travelling.
With wood, the bullet entry hole is, once again, much smaller than the diameter of the bullet. The wood fibres stretch and eventually tear as the bullet passes through. The majority spring back, making it extremely difficult to determine the calibre.
When dealing with wood, there is, however, a little-known method of determining the calibre with a reasonable degree of accuracy (Beta TAM Chi-Kung, unpublished work). If a piece of fairly strong white paper is placed over the wood surrounding the hole and a soft-lead pencil is carefully rubbed over the surface, as in brass rubbing, a circle, very closely approximating the diameter of the bullet, will appear.
Vehicle tyres are almost self-sealing, and it is often impossible to determine even the point of entry without immersing the tyre in water.
The determination of calibre from a bullet hole in a vehicle body can also be extremely difficult.
For example, semi-jacketed hollow-point bullets can expand on impact, giving the impression of a much larger calibre. At other times, the jacket material can be stripped off leaving the lead core to penetrate, giving the impression of a much smaller calibre.
Conversely, extremely high -velocity bullets, such as the 0.223" (5.56 mm) AR15 bullet can leave an extremely large entry hole. Often, there will also be a 'splash back' effect where metal flows back out of the hole, giving the impression it is an exit, not an entry hole. Identification of just the entry/exit holes in these circumstances can require considerable experience.
When high-pressure handgun cartridges loaded with plain lead bullets are fired through short-barrelled weapons, there is an additional problem. The problem here is that as the bullet emerges from the barrel, the gases which follow are still at an extremely high pressure. Once the bullet is freed from the constraints of the barrel, the pressure of the gases on its base is so great that it will expand. Sometimes, this base expansion can increase the diameter of the bullet by 50% or more, giving rise to a much larger entry hole than one would normally expect. In addition, the bullet can become unstable as the base expansion is not always constant. This can, in extreme cases, cause the bullet to tumble in flight.
In handguns, this phenomenon is only of any consequence with plain lead bullets fired in 0.357" and 0.44" Magnum weapons with a barrel length of less than 3" (Figure 4.30).
Rifles which have had their barrels shortened can exhibit this bullet base expansion with fully jacketed bullets as well.
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