A firearm magazine includes a housing defining a cavity. A resilient member is disposed in the cavity. A cartridge support member is movably supported on the resilient member and configured for moving downwardly into the cavity against increasing tension of the resilient member when the magazine is being loaded. The resilient member when under tension urges the cartridge support member and cartridges thereon to move upwardly during a feeding operation. An adapter converts a firearm magazine from multiple cartridge capacity to single cartridge capacity, and is configured to be removably coupled to the upper housing to cover the cartridge support member of the housing. The adapter includes an additional cartridge support member. The housing and the adapter cooperate to prevent the additional cartridge support member of the adapter from being moved downwardly into the cavity.