The CORE M has a built-in concealed AR15/M16/M4 Magazine Carrier that will accommodate all standard 5.56x45 (.223 Rem) AR15/M16/M4 and FAB Defense Ultimag Magazines. Whether its your accuracy or numerous target engagements, you can depend on your last ten chances with the FAB Defense "Survival" AR15 Butt-stock w/Built-in 10r Magazine Carrier. The CORE M AR15 Stock has a unique Anti-Rattle mechanism that holds the stock firmly in place on your AR15 tube eliminating any unnecessary rattle, furthermore the butt-stocks butt-pad in made out of a high-quality rubber to prevent it from slipping while shooting and enhancing the grip on the shooter's shoulder. The stock will fit both MIL-SPEC and Commercial tubes.