The Spectre M4 SMG has an interesting 4-stack magazine. Diagrams show the magazine as essentially two double-stack magazines converging together ( https://en.wikipedia.org/wiki/Spectre_M4 ). The patent diagrams are more detailed ( https://patents.google.com/patent/US4589218 ), but it would be helpful to see some pictures of the magazine in a disassembled state (which I have not found) to better understand how it functions.