AC556s were all MGs -- AFAIK, Ruger never built them as SBRs.
The factory shorty AC556 has a 13" barrel. The barrel and gas block are unique to the AC556 (different from the Mini14 counterparts) but they will fit a Mini14 that has been registered as an SBR, as will the Ruger factory folding stock.
Unfortunately, all three parts -- 13" barrels, gas block and folding stock -- are out of production and Ruger is out of inventory, with no plans to produce those parts again. So the only source would be a demilled used AC556 parts kit, if you can find one.
I've seen exactly one SBR'd Mini14 that had the barrel cut to 13" and an AC556 gas block and folding stock added to it. It was nice, but at more than $1k at a time when you could buy a full-auto AC556 for $5.5k.... undoubtedly due to the costs of the rare parts.