Magazines come in two varieties: fixed and detachable. Bolt-action rifles popular in World War I, such as the British Short Magazine Lee-Enfield Mk II "Smellie," serve as good examples of the former design. The SMLE incorporated a 10-round box magazine permanently fixed to the weapon. When a soldier fired all 10 rounds, he had to feed more ammunition into the box. Clips, which allowed five to 10 rounds to be inserted as a group, made reloading easier, but it still took more time than a soldier swapping one detachable magazine for another. For these reasons, assault rifles evolved to include detachable magazines as a standard feature.
As you might expect, state and federal laws regard rifles with fixed magazines more favorably than those with the detachable variety. And yet the gun industry found ways to work around the issue. For example, a rifle equipped with a magazine lock is perfectly legal. A lock requires a tool to remove the magazine from the firearm, which makes rapid reloading much more challenging. One popular lock is known as a bullet button because it releases the magazine only when the shooter fits a bullet or cartridge into its opening. Because the bullet acts like a tool, the magazine falls into the fixed category, and the rifle remains legal. Some gun-control advocates see this as a technicality and hope to include bullet buttons as a defining characteristic of assault weapons.