The 50 caliber (not 50 BMG) AR-15 by Alexander Arms. This is the current model which is a traditional direct gas impingement AR. They did announce a piston Beowulf back in January but its release date was pushed back to this fall. The Beowulf upper will fit onto most in-spec AR-15 lowers, though I’m not sure if carbon fiber lowers are okay for use.
Note the lack of a dust cover due to the enlarged ejection port. A custom dust cover can be purchased from a few aftermarket companies. The muzzle brake is the Alexander Arms “Pepperpot”.