| A magazine clamp is an accessory for detachable firearm magazines that allows the user to affix together multiple magazines. This means it's probably also a Terrifying Assault Weapon Thingy™.
The clamp facilitates reloading of firearms by reducing movement and actions performed for reloading. Normally extra magazines are stored in pouches worn on the body; when reloading, the user has to go through a large range of motion from pouch to firearm. Furthermore, depending on the user's stance, ammunition pouches may not be easily accessible (e.g. when prone).
However, when equipped with a magazine clamp, the wielder of the weapon only has to shift the depleted magazine left or right to the next fresh magazine.
Magazine clamps come in different varieties. There are actual clamps that consist of adjustable arms and a screw mechanism to tighten the arms. There are also magazine specific clamps that can only be used with certain magazines.
For the ill-prepared, magazine clamps are easily improvised in the field with some kind of binding material (usually some kind of cloth or electrical tape), and something to space the magazines apart so they do not interfere with each other when reloading.