Cross Industries Magazine, 556NATO, Fits AR-15, 10 Rounds, Contains Two 10-Round Coupling Magazines, Black CM10-AR15P-55645-BLK Featuring a unique coupling mechanism integral to the floor plate" couple two magazines end-to-end in 4 unique configurations in seconds" allowing faster reloads. The transparent bodies allow quick visual indication of how many rounds you have left" or the type of rounds" which is particularly useful for running 300 Blackout. The integral coupling floor plate allows you to couple the magazines end-to-end in either a curved or straight configuration; from there you decide if you want your cartridges facing forward/forward" or forward/reverse. If you ever change your mind about a configuration" change it seconds" without extra parts or special tools. Or un-couple the mags and take advantage of low-profile mags to shoot prone" on a bench or hunt in the woods.