Aluminium magazine base to replace your stock plastic one as they have an irritating habit of cracking and chipping over time. These caps are compatible with both Tokyo Marui and WE Hi-Capa magazines of both standard and long length varieties.
Besides offering greater durability to harm, they come in a variety of colours. Besides just spicing up your magazines, some users have been known to use the colours to colour code their magazines so that they can carry a variety of BB weights and gas pressure loads to mix and match performance on the fly.
Although not strong as steel, Aluminium is a little lighter and of course as a non ferrous material it will not rust. This is particularly important as gas magazines tend to have steel bodies and any rust that builds up on caps can risk the magazine itself where as Aluminium cap can act as a guard.