A fixed magazine in compliance with new firearms laws that enables citizens to legally own and effectively operate a firearm, such as an AR-15. The fixed magazine allows a user to easily load ammunition through a side of the fixed magazine without removing the magazine from the firearm, disassembling the action, or using tools. The fixed magazine includes: an extension portion configured to contain a stack of cartridges, and an attachable blocking tab for blocking removal of the ammunition magazine from the magazine well, the attachable blocking tab being attached after insertion of the ammunition magazine into the magazine well, and before re-engaging the upper receiver with the lower receiver. The fixed magazine also includes a cartridge-loading portion having a side opening, with a cover configured to alternately cover or reveal the opening, and a follower compartment including: a finger-pull slot with a bottom hook for locking the follower.