Cross Armory's Safe Mag is a magazine locking system modification, designed in California, for weapons in regulated states that require fixed magazines. It prevents the removal of the magazine from your rifle without disassembly of the action when properly installed. The Cross Armory Safe Mag is easy to install and works by releasing your magazine instantly when you separate the upper and lower receivers of your firearm (which disables the firearm action), without having to press a button. The Safe Mag also allows for insertion of a new magazine when the upper receiver and lower receiver are closed, without bending or stressing the hardware of the weapon or the Safe Mag. This Safe Mag is the 308 DPMS configuration which is compatible with 308/AR-10 models measuring 1-1/4" (1.25") wide at the base of the upper receiver (directly above the trigger). If it is 7/8" (0.875") wide, you will need the 308 Aero configuration.