Magpul Industries PMAG M3, 300BLK, 20 Rounds, Fits AR Rifles, Black MAG1182-BLK Magpul's PMAG 20 AR 300 B GEN M3 is a purpose-built magazine for rifles chambered in .300 Blackout (300 BLK). Similar in every way to our PMAG 30 AR 300 B" the PMAG 20 is more compact for increased maneuverability and creates a more concealable package for use in small format firearms.The PMAG 20 AR 300 B's distinct ribbed design and smoother upper-half texture provides the shooter a distinct visual and tactile difference from any other PMAG to mitigate dangerous cross-loading issues.