The XM177 Commando variant of the M16 rifle used a unique flash suppressor sometimes called a flash or sound moderator for its 10-inch barrel. This device is 4.2 inches long and was designed primarily as a counterbalance measure, as the shorter barrel made the weapon unwieldy. This device reduced flash signature greatly and sound signature slightly, making the normally louder short barreled rifle sound like a longer barreled M16A1. Although it has no internal baffles and does not completely reduce the sound signature to subsonic levels, because it alters the sound level of the weapon, the US Bureau of Alcohol Tobacco Firearms and Explosives has declared this device to be a sound suppressor and regulates its civilian purchase in the United States.