The 5.56x45mm NATO cartridge with the standard military ball bullet (NATO: SS109; U.S.: M855) will penetrate approximately 15 to 20 inches (38 to 50 cm) into soft tissue in ideal circumstances. As with all spitzer shaped projectiles it is prone to yaw in soft tissue. However, at impact velocities above roughly 2,700 ft/s (820 m/s), it may yaw and then fragment at the cannelure (the crimping groove around the cylinder of the bullet).[citation needed] These fragments can disperse through flesh and bone, inflicting additional internal injuries.[8] Fragmentation, if and when it occurs, imparts much greater damage to human tissue than bullet dimensions and velocities would suggest. This fragmentation effect is highly dependent on velocity, and therefore barrel length: short-barreled carbines generate less muzzle velocity and therefore lose wounding effectiveness at much shorter ranges than longer-barreled rifles. The rapid transfer of energy also results in wounding effects beyond the tissue directly crushed and torn by the bullet and fragments.[2][3] These remote wounding effects are known as hydrostatic shock.[4]
There has been much criticism of the poor performance of the bullet on target, especially the first-shot kill rate when using firearms that don't achieve the velocity to cause fragmentation.[9] This wounding problem has been cited in incidents beginning in the first Gulf war, Somalia, and ending in the current conflicts in Iraq and Afghanistan. In recent lab testing of M855, it has been shown that the bullets do not fragment reliably or consistently from round-to-round, displaying widely variable performance. In several cases, yawing did not begin until 7"-10" of penetration. This was with all rounds coming from the same manufacturer.[9] This lack of wounding capacity typically becomes an issue at increasingly shorter ranges (beyond 45m when using an M4 or 140m when using an M16 w/ a 20" barrel) or when penetrating heavy clothing, but this problem is compounded in shorter-barreled weapons. The 14.5-inch (37 cm) barrel of the U.S. military's M4 carbine generates considerably less initial velocity than its big brother, the 20" barreled M16 and terminal performance can be a particular problem with the M4.
From Dr. Roberts:[who?]
"Combat operations the past few months have again highlighted terminal performance deficiencies with 5.56x45mm 62 gr. M855 FMJ. These problems have primarily been manifested as inadequate incapacitation of enemy forces despite their being hit multiple times by M855 bullets. These failures appear to be associated with the bullets exiting the body of the enemy soldier without yawing or fragmenting. This failure to yaw and fragment can be caused by reduced impact velocities as when fired from short barrel weapons or when the range increases. It can also occur when the bullets pass through only minimal tissue, such as a limb or the chest of a thin, malnourished individual, as the bullet may exit the body before it has a chance to yaw and fragment. In addition, bullets of the SS109/M855 type are manufactured by many countries in numerous production plants. Although all SS109/M855 types must be 62 gr. FMJ bullets constructed with a steel penetrator in the nose, the composition, thickness, and relative weights of the jackets, penetrators, and cores are quite variable, as are the types and position of the cannelures. Because of the significant differences in construction between bullets within the SS109/M855 category, terminal performance is quite variable—with differences noted in yaw, fragmentation, and penetration depths. Luke Haag’s papers in the AFTE Journal (33(1):11-28, Winter 2001) describe this problem."[9] However, if the bullet is moving too slowly to reliably fragment on impact, the wound size and potential to incapacitate a person is greatly reduced. Several alternate cartridges have been developed in an attempt to address the perceived shortcomings of 5.56mm ammunition including the 6.5 mm Grendel and the 6.8 mm Remington SPC.
Recently, advances have been made in 5.56mm ammunition. The US military has adopted for limited issue a 77-grain (5.0 g) "Match" bullet, type classified as the Mk 262. The heavy, lightly constructed bullet fragments more violently at short range and also has a longer fragmentation range.[10] Originally designed for use in the Mk 12 SPR, the ammunition has found favor with special forces[citation needed] units who were seeking a more effective cartridge to fire from their M4A1 carbines. It should be noted, however, that commercially available loadings using these heavier (and longer) bullets can be prohibitively expensive and cost much more than military surplus ammunition. Additionally, these heavy-for-caliber loadings sacrifice even more penetrative ability than the M855 round (which has a steel penetrator tip). Where M855 currently struggles to penetrate thin car doors and wood framed obstacles, the heavy bullets will do little more than spray the concealed party with bullet fragments.
Performance of 5.56x45mm military ammunition can generally be categorized as almost entirely dependent upon velocity in order to wound effectively. Heavy OTM bullets enhance soft tissue wounding ability at the expense of hard-target/barrier penetration.
