The effects of the 5.56x45 NATO ammuntion depends greatly upon its velocity at the time of impact against the target. 5.56 ammunition, if travelling at a high enough velocity, tends to fragment upon impact of a soft target. The bullet will penetrate into the body for about 4 inches, while it yaws. When it yaws, if it is travelling fast enough, the bullet cannot withstand the stress upon it, and it will fragment into many smaller pieces. These smaller pieces each travel outwards from the fracture point, multiplying the effect of the wound. The threshold for this fragmentation effect is 2700fps.
|Velocity (55gr M193)|
Feet per second
|Velocity (62gr SS109)|
Feet per Second
|Energy (55gr M193)|
|Energy (62gr SS109)|
As one can see from the table, anything under 10" in length will not provide enough muzzle velocity to attain the best wounding capabilities of the 5.56x45 projectile. Anything lower, and the bullet will make a .22 caliber hole through the target, with relatively insignificant wound ballistics (compared to the same bullet at 3100fps). This is why short barreled weapons like the HK53 and G36C are somewhat unsuitable for CQB (8" barrels, estimated muzzle velocity around 2550 fps). There have been reports of weapons of this barrel length to be unable to penetrate soft kevlar vests, which is very worrying for a weapon of rifle caliber.
One must also contend with the increased muzzle blast from such short-barreled weapons, which can be considerable. There have been numerous police reports that officers firing short-barreled AR-15 carbines (11.5") in small rooms have come out with bleeding ears and noses. This is why one often sees suppressed 5.56 weapons - although the weapon itself is relatively loud for a suppressed weapon, the suppressor keeps the muzzle blast down to a managable level. Also, with AR-15 variants with barrels shorter than 10", it has been found that they rarely perform reliably without a suppressor or a very long flash suppressor - which defeats the purpose of the short barrel. CAR-15 carbines of Vietnam had 10" barrels; however, they also had 4 inch long flash suppressors. When a normal A1 'birdcage' style flash suppressor was used on the CAR-15's, the carbines turned into very sexy looking single shot weapons - the short barrel does not allow enough back pressure to develop, which does not allow the gas operation system to work properly.
However, a new report in the most recent edition of Guns and Weapons for Law Enforcement has a very interesting article on super-short carbines for CQB use. These are AR-15 type weapons with barrels no more than 6 to 8 inches long. Although I dismissed these weapons as wishful thinking when I first saw them, they may have a very practical use in two to three years. Special types of very fast burning propellants are under development that allow not only muzzle velocities comparable to full-length AR-15 rifles, but the same reliability as well! The new ammunition, however, does not perform well in full-sized rifles, as the powder burns too quickly for reliable cycling. I shall post more information as I recieve it.