Minimal Velocities Necessary to Perforate Skin

Before a bullet can cause a significant injury, it must be able to perforate skin. Skin differs from other tissue in that a relatively high initial velocity is necessary for a bullet to effect perforation. Knowledge of this velocity is important to the forensic pathologist in cases of assault, attempted homicide, or homicide with airguns as well as in determining the maximum range out to which a bullet is capable of penetrating the body.

The first person to attempt to determine the minimum velocity needed to perforate skin was Journee in 1907.4 He observed that missiles of relatively low velocity (80 to 200 m/sec) that rebounded from the skin of a horse could go through 20 cm of muscle after the skin had been removed. Thus, skin appeared to be more resistant to missiles than muscle. Experiments on human cadavers revealed that a lead sphere 11.25 mm in diameter and weighing 8.5 g needed a minimum velocity of 70 m/sec (230 ft/sec), with an energy/area of presentation (E/a) of 2.13 m-kg/cm2, to perforate the skin and enter the underlying subcutaneous tissue and muscle.

Matoo et al. in 1974 obtained virtually the same results using human thigh muscle with intact skin.5 A lead sphere 8.5 mm in diameter and weighing 4.5 gm required a velocity of 71.3 m/sec (234 ft/sec) to perforate skin and penetrate into subcutaneous tissue and muscle to a depth of 2.9 cm. The E/a was 2.06 m-kg/cm2.

Both these studies involved relatively heavy large-caliber lead balls and not the lighter weight, bullet-shaped projectiles fired in modern firearms or the very lightweight projectiles used in airguns.

DiMaio et al. conducted a series of tests to determine the velocities necessary for .38-caliber lead bullets and lead airgun pellets (calibers .177 and .22) to perforate skin.6 Human lower extremities were used in the tests. A 113-gr. lead roundnose .38- caliber bullet required a minimal velocity of 58 m/sec (191 ft/sec) to perforate skin (Table 9.1). The E/a was 1.95 m-kg/cm.2

Caliber .22 wasp-waist Diabolo-style airgun pellets weighing an average of 16.5 gr. initially perforated skin at 75 m/sec (245 ft/sec), with perforation becoming consistent at 87 m/sec (285 ft/sec) and above (Table 9.1). The E/a at 75 m/sec was 1.30 m-kg/cm2. At a velocity of 68 m/sec (223 ft/sec), a pellet embedded itself in, but did not perforate, the skin.

0.177 airgun pellets of wasp-waist Diabolo style weighing an average of 8.25 gr. required a minimum velocity of 101 m/sec (331 ft/sec) to initially perforate skin (Table 9.1). At velocities of 111 m/sec (365 ft/sec) and higher, perforation always occurred. At a velocity of 88 m/sec (290 ft/sec), a pellet embedded itself in the skin. The E/a at 101 m/sec (331 ft/sec) was 1.84 m-kg/cm2.

Table 9.1 Minimum Velocities Necessary to Perforate Skin

Weight

Minimum Velocity

Missile

(gr)

(m/sec)

.177 airgun pellets

8.25

101 (331 ft/sec)

.22 airgun pellets

1B.5

75 (245 ft/sec)

.38-caliber round-

113

58 (l91 ft/sec)

nose bullet

McKenzie et al. conducted a series of experiments involving the firing of 7.9 gr., .177 caliber, airgun pellets, pointed and blunt tipped, at a newly killed (within 10 minutes of experimentation) pig. The pointed tip pellets had a velocity of perforation of 384 +/- 4 ft/s; the blunt tip pellets 403 +/- 3 ft/s.7

These studies indicate that lightweight projectiles need a higher velocity to perforate skin than large caliber heavier bullets.

Now that we have an idea of the minimum velocity necessary for bullets and airgun pellets of different weights and calibers to perforate skin, we must ask whether the missiles lose this velocity in perforating the skin. The answer is no. In an unpublished extension of the previously mentioned study, DiMaio and Copeland conducted a number of test firings using a human lower extremity to determine how much velocity was lost by a missile passing through the thigh.8 The bullets had to pass through two layers of skin and approximately 6 in. of muscle. Two different calibers of ammunition were used — .38 Special and .22 Long Rifle. In the tests with the .38 Special ammunition, two different types of ammunition were used. The first type was loaded with a 158-gr. lead round-nose bullet. Average impact velocity was 766 ft/sec. On an average, these bullets lost 280 ft/sec (36.8% of impact initial velocity) in passing through the thigh (Table 9.2). The velocity lost ranged from a minimum of 214 ft/sec to a maximum of 337 ft/sec.

Table 9.2 Velocity Lost by Bullets Perforating Human Skin and Muscle3

Bullet

Average

Range

Weight

Velocity Lost

of Velocity

Velocity

Caliber

(gr)

Bullet Style

(ft/sec)

Lost (ft/sec)

Lost (%)

.38 Special

158

Lead roundnose

280

214-337

36.8

158

Semi-jacketed

305

264-355

34.4

hollow-pointb

.22 Long Rifle

40

Lead roundnose

195

187-202

18

3B

Lead hollow-point

491

431-599

45.5

a Two layers of skin, 6 in. of muscle. b This bullet did not mushroom.

a Two layers of skin, 6 in. of muscle. b This bullet did not mushroom.

The second type of ammunition was loaded with 158 gr., semi-jacketed hollow-point bullets. The average impact velocity was 884 ft/sec. With this velocity and weight of bullet, there is no mushrooming of the projectile in the body. Therefore, mushrooming was not a factor in loss of velocity. The average velocity lost was 355 ft/sec for an average loss of 34.4% of impact velocity (Table 9.2). The velocity lost ranged from a low of 264 ft/sec to a maximum of 335 ft/sec. The increased loss of velocity by the semi-jacketed hollow-point bullet compared with the roundnose bullet, if significant, could be due to either one or the other of two factors if not a combination. The first factor is the greater velocity at which the semi-jacketed bullet was propelled and the second is the blunt shape of the tip necessitated by having a hollow point. Mushrooming of the bullet did not occur and therefore could not play a part in an increased loss of velocity. In all probability, the greater impact velocity caused the greater loss of velocity. This theory tends to be confirmed by the fact that the percentage loss of impact velocity for both styles of bullets was approximately the same.

The tests with the .22 ammunition were somewhat more extensive in that the loss of velocity was determined not only for the thigh when it was enclosed by skin but also for the muscle alone. This was accomplished by the removal of the skin after test-firing with it in place. The first ammunition tested was high-velocity .22 Long Rifle cartridges loaded with 40-gr. lead roundnose bullets. The average impact velocity was 1083 ft/sec. Average loss of velocity in passing through the thigh was 195 ft/sec, with velocity lost ranging from 187 to 202 ft/sec (Table 9.2). When the skin was removed from the thigh, this same ammunition lost an average of 151 ft/sec range (85 to 229 ft/sec). Thus, in passing through two layers of skin, the bullets lost only an average of 44 ft/sec.

The second type of ammunition was high-velocity .22 Long Rifle ammunition loaded with a 36-gr. lead hollow-point bullet. The average striking velocity was 1079 ft/sec. Average velocity loss was 491 ft/sec with a range of 431 to 599 ft/sec, approximately 2 1/2 times the velocity lost by the solid roundnose bullets (Table 9.2). When the hollow-point ammunition was tested against the thigh with the skin removed, there was an average loss of velocity of 383 ft/sec, with a range of 320 to 520 ft/sec. Thus, in passing through two layers of skin, the hollow-point bullets lost an average of only 108 ft/sec. The increased loss of velocity in passing through the skin compared with the solid lead bullets is consistent with the increased loss sustained while passing through muscle.

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