Magpul manufactures 300 BLK specific mags but only in the 30 round version. What limited information I have found says the standard AR15 mags are only an issue with the larger bullets and work fine with the smaller bullets, the problem being the rib in the magazine hits the larger bullet and causes it to shift in the mag.
Anyone have any direct experience with this? It would be nice to use 10 rounders, especially for bench shooting where the longer ones may interfere.
D&H recently came out with 300 BLK specific magazines in 10, 20, and 30 round versions.
I've been using some 10 and 20 round versions off the bench (30's are a little too long) and they have worked with zero problems so far.
For giggles I purchased the Magpul 30 and all three sizes of the D&H. They work well. I only had an issue with the 20 round D&H as it wouldn't lock in. I've sent it back for a replacement.