While most current AR15 based developments are based on opening up the existing AR15 .223 bolt face to accommodate a larger cartridge, we have taken a different approach and use a larger bolt. As you can see in the photo below the AR bolt is opened up for the 6mm AR. Compare that to the size of the BR15 bolt. As you can see it is significantly larger and stronger to meet the needs of the BR family of cartridges.
.223 Bolt 6mm AR Bolt BR15 Bolt
Along with the larger BR bolt the barrel extension is also larger. This allows just a little more barrel material around the chamber than the .223 and quite a bit more than is available with the 6mm AR (7.62x39 based), or any of the 6.8 SPC/.30 Rem based cartridges.
AR15 Barrel Extension BR15 Barrel Extension
Cartridge Cart. Size Barrel Extension Thickness between chamber and barrel extension
.223 Rem .376" .810" .217"
6.8 SPC .421" .810" .1945"
7.62x39 .441" .810" .1845"
6BR .470" .910" .220"