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Monty, All 30 round AR magazines function with 300 BLK. Have never used this 60 rounder. Hickok had problems with this magazine on a AR review. Should function just fine with 300 BLK.

Monty, no Sir. I have several of these Surefire Magazines and mine are not reliable with 300BLK. I believe it has to do with the weight difference bullet feed angle between the 300Blk round compared to the 5.56/223 which this magazine 's feed system was designed specifically for 5.56/223. I get only 75 to 80% reliability with 300BLK and 100% so far with 5.56/223.