BDX

Similar to a base plate, a base pad secures the internal components of a pistol magazine. Pistol base pads can be made of a variety of metals or polymers. Some base pads are constructed with extra room to allow shooters to add more rounds to their standard magazine. Other base pads are constructed of brass or other metal alloys to add weight or texture to the magazine, helping competitive shooters execute quicker reloads. https://www.sigsauer.com/electro-optics/bdx.html