.300 Blackout Rifle Brass Headstamps Are Mixed 300 Blackout with up to 20% .223/5.56 Unprocessed Brass for Reloading Washed and Polished Range Brass 100% Brass Casings 1000+ Casings .300 Blkout Brass is a mix of factory original .300 Blackout cases and converted 5.56/.223 cases. The 300 AAC Blackout case is ideal for subsonic and supersonic loads, in both suppressed and unsuppressed firearms. By utilizing modified 5.56/.223 cases, the .300 Blackout is compatible with standard AR|15 magazines and bolt carriers, making it an accessible and cost|effective choice for shooters.