If you shoot 300 BLK out of an AR platform (or a rifle that accepts AR pattern mags) then you should take a look at the PMAG 20 AR 300 B GEN M3, 300 BLK which are now shipping from Magpul.
One of the draws to 300 BLK is that it works well in a compact carbine configuration and this new 20 round mag is 2″ shorter than its 30 round bigger brother, giving just a little bit more advantage for use in tight spaces.
These magazines are specifically engineered and marked for 300 BLK use.
magpul.com/pmag-20-ar-300-b-gen-m3