A multi-caliber automatic rifle (AR) magazine is configured to alternately receive a first cartridge having a first caliber and a second cartridge having a first caliber. The multi-caliber AR magazine includes a first pair of ribs, a second pair of ribs, and a third pair of ribs such that the first pair of ribs and the second pair of ribs are configured to both support the body of the cartridge when loaded in the magazine. The second pair of ribs are indented toward a center of the multi-caliber AR magazine housing more than the first pair of ribs such that a distance between the second pair of ribs is less than a second distance between the first pair of ribs. The third pair of ribs can be rebated to form a dual-curved rib having a central convex portion connected between two concave portions.