Arne Brennan, creator of 6.5 Grendel and other cartridges, designed 375 Raptor in a quest to do for 308 Winchester what 300 Blackout did for .223 Remington. Namely, neck it up to run larger, heavier bullets that deliver both supersonic and subsonic performance in a cartridge that’s efficient from a short barrel, suppresses well, and runs reliably in all the same platforms as its parent cartridge does.
By necking up and trimming 308 Winchester brass, the parent case is made ready to accept a .375 caliber projectile.