1 Get an M4 with a 2 slots magazine in it (like a 30 or 60 butter magazine).
2 Get a (full) 10 bullet magazine in your inventory (1 slot)
3 Fill up ALL the other inventory space so there is no slot left.
4 Press F
5 An option will be available to swap magazines, press that
6 Big magazine will be gone because there was not enough room in the inventory.