The .300 Blackout cartridge is mostly compatible with rifle parts intended for 5.56 NATO, but using .300 Blackout rounds in a 5.56 NATO magazine can lead to multiple issues. .300 Blackout rounds have a slightly different geometry than 5.56 rounds, which can cause the rounds to incorrectly “stack” when loaded into 5.56 magazines. This leads to feeding failures, limits the capacity, and can be harmful to the 5.56 magazine itself. Using .300 Blackout rounds in a 5.56 magazine also presents the serious possibility of accidently chambering a .300 Blackout round in a 5.56mm rifle with catastrophic results, and requires diligent ammunition management by the shooter. Magpul solves all of these issues with the PMAG 30 AR 300 B GEN M3, a magazine built from the ground up for use with the .300 Blackout cartridge. The PMAG 30 AR 300 B GEN M3’s internals have been optimized specifically for the .300 Blackout round’s geometry, ensuring flawless feeding and reliability. The magazine also features visual and tactile indicators to minimize the possibility of dangerous cross-loading. It utilizes a distinctive rib design as well as a smooth upper half, allowing the user to instantly identify the magazine upon picking it up. The PMAG 30 AR 300 B GEN M3 also features a black follower, and paint pen dot matrix on the bottom portion allows the user to mark the magazine for visual identification. Features of the PMAG 30 AR 300 B GEN M3:Specifically designed for the .300 Blackout cartridge Internal geometry optimized for .300 Blackout rounds Visual and tactile indicators for instant identification – minimizes risk of dangerous cross-loading Magpul 20rd Dedicated .300BLK AR Pattern PMAG GEN M3 , Black MAG1182-BLK