A box magazine for a firearm may have a housing with two substantially different widths. A wider region may accommodate four columns of cartridges and may have tapered surfaces at one end that lead into a narrower region which may accommodate an interlocked and staggered double column of cartridges. Cartridges in the wider region may be separated into two staggered double columns by a free standing dividing wall and may be advanced by inner and outer contoured followers that may be nested together to form a single follower assembly surrounding the freestanding wall and that may be driven by the outer follower's spring. An inner follower spring may remain nested within the outer follower such that the two followers coordinate the advance of the two separated double columns of cartridges toward tapered surfaces that merge the two double columns into two single columns proximate the top of the dividing wall.