<P> This failure of 5.56 × 45mm NATO bullets to yaw can be caused by reduced impact velocities as when fired from short - barreled weapons or when the range to the target increases . Failure to yaw and fragment can also occur when the bullets pass through only minimal tissue, such as a limb or the chest of a thin, small statured individual, as the bullet may exit the body before it has a chance to yaw and fragment . Two other yaw issues: Angle - of - Attack (AOA) variations between different projectiles, even within the same lot of ammo, as well as Fleet Yaw variations between different rifles, were elucidated in 2006 by the Joint Service Wound Ballistic Integrated Product Team (JSWB - IPT), which included experts from the military law enforcement user community, trauma surgeons, aero ballisticians, weapon and munitions engineers, and other scientific specialists . These yaw issues were most noticeable at close ranges and were more prevalent with certain calibers and bullet styles--the most susceptible being 5.56 × 45mm NATO FMJ ammunition like SS109 / M855 and M193 . </P> <P> The 5.56 × 45mm NATO standard SS109 / M855 cartridge was designed for maximum performance when fired from a 508 mm (20.0 in) long barrel, as was the original 5.56 mm M193 cartridge . Experiments with longer length barrels up to 610 mm (24.0 in) resulted in no improvement or a decrease in muzzle velocities for the SS109 / M855 cartridge . Shorter barrels produce a greater flash and noise signature, and the addition of a suppressor to a short barreled AR family rifle can make it unreliable, as the reduced time for the propellant to burn in the barrel and higher muzzle pressure levels at the suppressor entrance can cause faster cycling and feeding issues . Unless the gas port can be regulated or adjusted for higher pressures, suppressors for short barreled 5.56 × 45mm NATO firearms must be larger and heavier than models for standard length rifles to function reliably . SS109 / M855 cartridges fired from barrels under about 254 mm (10.0 in) in length do not have enough muzzle velocity energy to cause extreme damage that occurs only at terminal velocities of over 750 m / s (2,500 ft / s) on impact, reducing the wounding capacity . </P> <P> Compared to larger calibers, proponents of the 5.56 × 45mm NATO round contend that animal studies of the wounding effects of the 5.56 × 45mm NATO round versus the 7.62 × 39mm have found that the 5.56 × 45mm NATO round is more damaging, due to the post-impact behavior of the 5.56 mm projectile resulting in greater cavitation of soft tissues . The US Army contended in 2003 that the lack of close range lethality of the 5.56 × 45mm NATO was more a matter of perception than fact . With controlled pairs and good shot placement to the head and chest, the target was usually defeated without issue . The majority of failures were the result of hitting the target in non-vital areas such as extremities . However, a minority of failures occurred in spite of multiple hits to the chest . </P> <P> Some have contended that shot placement is the most important parameter in determining the lethality of a bullet . Difficulty with the 5.56 × 45mm NATO at long ranges has been attributed to training; Swedish ISAF units relied on . 50 BMG heavy machine guns for long - range shooting due to resilience to factors such as range and wind drift . Underperformance is thus attributed to errors in range and wind estimation, target lead, firing position, and stress under fire, factors that can be resolved through training . </P>

Why does the military use the 5.56 round