Why use a Magpul PMAG® 30 AR 300 B Gen M3TM Mag when a 5.56 Mag will work with your .300 BLK ammo? It's all about the stack. This specially designed .300 BLK Mag eliminates 5.56 mag control points that don't interface correctly with .300 BLK ammo. That means you get a proper stack, with ammo feeding correctly with little side pressure and less friction for more reliable shooting than you would experience with a 5.56 mag. And it doesn't limit the capacity of your Mag, which can happen when loading .300 BLK into a 5.56 mag. You'll fit 30 rounds of .300 BLK in these mags, and you'll enjoy flawless shooting, too. Don't settle for limited performance for your .300 Blackout rifle, maximize it with these Mags instead.