From Small Arms Defense Journal:

The 300 Blackout cartridge fits in the standard AR magazine without any loss of the original 30 round capacity.  The standard bolt and barrel extension is used as well.  The only difference is the barrel itself.  This not only makes for easy logistical support for potential military use of the caliber, but you can pull spare parts from existing .223/5.56mm rifles.  The sample rifle has been tested with GI aluminum magazines, H&K high reliability mags, Lancer AWM and Magpul PMags.