The new U.S. Assault Rifle, the XM8, is a modular weapon that can be fitted with three different barrels (9", 12.5", 20".) The 9" barrel is for a very compact weapon for tank crews or commandoes. With the nine inch barrel and the butt stock retracted, the weapon has an overall length of 21 inches. The 12.5" barrel is the standard, for use with the assault rifle or, with the butt stock retracted, it serves as a carbine. The 20 inch barrel is thicker as well and used for the light machine-gun version. There is also a lighter 20 inch...