I built a 7.5" 5.56mm SBR as a test gun for our drug task force. The purpose was for a vehicle-borne operations carbine. It was basically a worthless project that created nothing but headaches. Literally. The pressure from the muzzle was tremendous and gave people sinus headaches. Ballistically, the only load that worked out of it was the 45gr TSX.
If you want a super short SBR, it would be ideal to use a caliber that is better suited to lower velocities like the .300BLK or 6.8SPC. The Federal XM68GD 90gr BJSP was specifically designed for the 8.5" barrel LWRC Six8 UCIW.