The caliber of the bullet that caused an entrance wound in the skin cannot be determined by the diameter of the entrance. A .38-caliber (9-mm) bullet can produce a hole having the diameter of a .32 caliber (7.65-mm) bullet and vice versa. The size of the hole is due not only to the diameter of the bullet but also to the elasticity of the skin and the location of the wound. An entrance wound in an area where the skin is tightly stretched will have a diameter different from that of a wound in an area where the skin is lax. Bullet wounds in areas where the skin lies in folds or creases may be slit-shaped.
The size of an entrance hole in bone cannot be used to determine the specific caliber of the bullet that perforated the bone though it can be used to eliminate bullet calibers. Thus, a bullet hole 7.65 mm in diameter would preclude it having been caused by a 9-mm (.38 caliber) weapon. Bone does have some elasticity, however, so that a 9-mm bullet may produce a 8.5-mm defect.
In the author's experience, the size of a bullet hole in bone is determined not only by the diameter of the bullet but also by its construction. Entrance holes of the temporo-parietal region from .25 ACP (6.35-mm) and .22 (5.45mm) rimfire bullets were compared. The .25 ACP bullets were of full metal-jacket design while the .22 bullets were of lead. The holes in the bone from the .25 ACP bullets averaged 6 to 7 mm in diameter. In contrast, the entrance holes from the lead .22 caliber bullets ranged in diameter from 5 to 11 mm. The only major difference between the two calibers is the construction of the bullets: full metal-jacketed versus lead. The lead bullets expanded to some degree on impacting and perforating the skull, thus, producing the larger entrance holes.
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