Most bullets make small sonic booms when flying through the air, which to our ears sound like a loud, distinct “crack!” For the Pentagon‘s special forces, that makes it hard to be sneaky about what they’re shooting. Now the commandos want to be sneakier with slower, quieter bullets.
In its latest round of small-business solicitations, the Pentagon’s Special Operations Command, or SOCOM, is seeking out subsonic ammunition. The reason, according to the solicitation, is to “provide superior covert and stealth capabilities” for not only the military, but police forces and the Department of Homeland Security. In theory, and for rifles in the 5.56, 7.62 and .338 calibers, the bullets will travel at low enough velocities to avoid breaking the sound barrier, thus creating no “crack” noise. Breaking the sound barrier also pretty much negates the use of a sound suppressor, or “silencer,” which the special forces would likely want to use against militants in Afghanistan and around the world.
At present, the Defense Department does not have subsonic bullets “classified for use in the calibers provided by any DoD service.” That doesn’t mean special operations forces never use them. Commandos have used subsonic bullets since World War II, though these are mainly effective in smaller guns like the .22 and 9 mm caliber pistols. Subsonic bullets and fairly large-caliber war rifles, on the other hand, don’t mix very well.
For one, to keep a bullet from breaking the sound barrier — 1,100 feet per second at sea level — requires several trade-offs at higher calibers. According to the solicitation, subsonic bullets “experience significant accuracy problems due to excessive deviations in velocity.” The gunpowder (or propellant charge) for a subsonic bullet has to be used in smaller quantities than for a normal bullet, and the bullet itself has to be heavier. This results in bullet that is far and away less accurate, doesn’t go nearly as far, and “creates lower pressures which … makes it hard to get a clean burn of the propellant causing rapid fouling of the weapon.”
In technical jargon, the failure of a clean “burn” and the resulting lowered accuracy and range is called a failure of “obturation.” Normally, a bullet expands — or obturates — to the size of its barrel after being fired, keeping the bullet on target and preventing the gases that propel it from rushing past and melting to the inside of the gun. The melted leading can be a pain in the ass to remove, and can permanently damage the weapon unless it’s cleaned. Likewise, a bullet can’t be too light or too heavy, because too much in either direction can prevent the bullet from obturating. Using subsonic bullets also causes a greater risk of jamming, which in a firefight could mean the difference between life and death.
Instead, the Pentagon has one idea about how to build a better subsonic bullet. One solution could be using “polymer cased ammunition” as opposed to brass or steel. The Pentagon is somewhat vague about how this will work, but the idea is that polymer-cased bullets “produce a reliable and consistent powder burn.” More specifically, polymer obturates at lower pressures, which means it may be possible to shoot a heavy bullet with less propellant while theoretically not trading for accuracy and range. Maybe.
To do it, the Defense Department might want to go back to the future. During the late 1980s and early 1990s, the Army spent $300 million on a canceled project called Advanced Combat Rifle to replace the standard M16 rifle. One proposed replacement, the Steyr ACR, used polymer cartridges, but supposedly suffered from inaccuracy due to the strength of the cartridges being inconsistent, though this could be conceivably solved by testing cartridges until they fire consistently. Perhaps SOCOM could do it better.