Even if a gun comes off the line legal, it can be modified into something that pushes it into assault-weapon territory. Obviously, if a gun owner installs a grenade launcher on her AR-15 rifle, she's crossed the line. But other modifications venture into murkier water. One such modification is a replacement stock for AR-15 rifles that enables the operator to "bump fire" the gun, more safely, while it's held against the shoulder. "Bump firing" refers to a technique used by semi-automatic gun owners who want to shoot their weapon in near-automatic mode. To pull this off, someone holds the rifle at hip level, with the trigger finger held firm just in front of the trigger, and then presses the gun forward with the opposite hand. This causes the trigger to bump into his finger, which discharges the weapon and makes it recoil, which propels the rifle back against the trigger finger, allowing the gun to fire rapidly.
This technique requires a lot of practice and, in reality, poses a serious challenge to most novice gun owners. That's where a slide fire stock comes in. After a five-minute installation, this simple device provides the bump-firing experience while the weapon is held at the shoulder, where it can be operated more safely. Early versions contained springs, but the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives soon banned them. Newer versions got around the ban with a design that allows the device to slide back and forth on the rifle's buffer tube while the non-trigger hand supplies all of the spring action. Equipped with a slide fire stock, a semi-automatic rifle fires almost as rapidly as a machine gun, which is why new U.S. assault-weapon legislation is hoping to ban this type of modification in all its forms.