This is ProMag's innovative 40 round RollerMag magazine for AR-15 rifles in .223 5.56x45 and .300 AAC Blackout.
ProMag have mounted rollers on their anti-tilt follower to provide a very smooth and reliable feed, with overwhelmingly positive reviews.
Compatible with all AR-15 platform rifles.
Please note: will not fit the SA80 / L85A2.