H&K UMP 120-Round Midcap AEG MagazineMid capacity magazines hold between 80 and 150 rounds. They use the same mechanism that low capacity magazines use, except they are able to hold more BBs. They use a long serpentine spring wound through a channel to hold and then push the BBs out. They are a very simple design and less prone to failure than hicaps. Midcaps are useful in that they dont rattle like hicaps, but hold more BBs than lowcaps. Mid capacity magazines are the favorite of advanced players since they tend to test their aiming and trigger control abilities. The easiest way to load these magazines is with a speedloader.