A friend was telling me about an experience he had in the Phillipines, where CIA agents had quad stack magazines. I told him that I had heard of them, but only remembered that the were of Scandinavian origin. A little research revealed that they were for the Soumi K31 submachine gun, made in Finland. Several other submachineguns were made that use the magazine. A fair number of Swedish K (M45) submachine guns can use them. The Swiss M43/44 is a licensed copy of the Suomi, as is the Swedish/Danish M37/39 Here is an excellent YouTube video describing the magazine...