It may be something entirely different.
Key point: Technically, the AR-556 is an AR-15 pistol. That means it is designed to be smaller but still packs a big punch.
Ruger’s AR-556 pistol, a short-barreled AR-15 designed for one or two-handed operation, delivers rifle-level performance in a pistol-sized configuration.
The AR-15 rifle, or ArmaLite Rifle-15, was developed by Eugene Stoner as a potential military firearm. The AR-15, first adopted by the U.S. Air Force and later by the rest of the armed forces, is a gas operated, direct impingement firearm that siphons off gunpowder gasses to cycle the weapon. This also has the beneficial side effect of significantly reducing recoil, allowing a rifle shooting a 55-grain cartridge at velocities in excess of 3,000 feet per second to be easily manageable.
Modifications to the AR-15 gas system can also allow for the use of very short barrels, barrels technically shorter than those allowed under the Federal National Firearms Act (NFA). The NFA identifies rifles with barrel lengths shorter than sixteen inches as subject to special regulation, typically involving additional paperwork and a tax stamp. Such “short barrel rifles” are also illegal in many states and municipalities, further complicating their purchase.
A popular workaround to the NFA is the classification of a weapon as a pistol instead of a short barrel rifle. Pistols are not regulated by barrel length. That having been said, pistols do not have buttstocks and are typically not fired from the shoulder. The Bureau of Alcohol, Tobacco, and Firearms however recently declared arm braces on AR-15 “pistols” are in fact legal, even when the “braces” are incidentally capable of firing (though not comfortably) from the shoulder.