What gun has the biggest magazine?

The Ultimax light machine gun has a standard 100 round drum magazine. It's a bit of a difficult question to answer, as magazines larger than the standard have been made for a number of weapons system. For example, the 100 round Beta-C magazine uses a STANAG 4179 (M16 magazine) interface, and is thus compatible with any weapon system which uses this magazine, but it is not a standard item.