Living in CT I am stuck with the one Sig SCM that I registered.  I added the swiss folder lower but am wondering if I can convert it to the 551 SBR short forearm and barrel?  Probably rare with the pistol 556 option everywhere else.  But is it possible and does anyone do it?

Shouldn't be a issue. I'm sbring my 556 patrol that I put a 551 lower on. As long as your keeping the upper receiver seeing as how that is your registered part. I may be wrong I'm not sure on ct laws. With the short 551 forearm are you talking USA 551 or Swiss 551

I was talking US style. I want my first SBR and the Sig shorty has always been my favorite.  Did your gun start with a long hand-guard or was it originally short?

The patrols are the short, and I'm cutting mine down to 11.4in. If you want to convert to a short forend there is a few ways you can do it. You can buy a demilled USA sbr upper like the one in this link. http://www.gunbroker.com/Auction/ViewItem.aspx?Item=462751458
then you will have everything you need otherwise you need to find a patrol barrel (if you want one longer than 10.5) with the matching gas block (preferred) and the pistol or patrol gas system. If you like the factory plastic forend like myself and you can't find one for the price you want to pay you can cut down your factory plastic to fit.

If you want I can provide a picture of mine would look like with a 11.4in barrel