I'm not sure if this will help, but I have an HK51 .308 with an 8.3" barrel. The recoil is actually less than with a 16" HK91 barrel because so much more velocity is lost compared to weapon weight lost. I would assume then that the weapon gets beat up less as the BCG is moving at a lower velocity. Functioning was not affected, but the HK uses a competely different operating system than the AR. I think you might have a little trouble with getting the correct gas pressure because IIRC most of the .308 AR's use a mid-length gas system when a carbine length would probably be better for a SBR.
Ammunition for the naysayers: When I shoot quality (Hornady TAP) ammunition through the HK51, muzzle flash is not significant and accuracy is very acceptable. I am able to shoot 3.5-4" ten shot groups at 100yds prone using standard iron sights. It is VERY loud, though, and muzzle blast is significant.