I recently fired a Sig 556 short barrel rifle and am interested in one day buying my own.
The one that I shot was built at the factory as a short barrel rifle. I have seen a number of these that began as 556 pistols that were converted to rifles. They are significantly less expensive than the factory short barrels. Are there any quality or performance differences between factory SBR's and pistol-to-rifle SBR conversions?
pistol to sbr will require engraving and the factory sbr won't. otherwise same same

I've read that if you convert a pistol to SBR that you will loose the warranty on it. Just to let you know before hand.
Personally I really doubt that you would even need the warranty on it. Tough little guns.