As long as it's manufactured as a short barrel and not cut down you're good to go. Dlask makes 12" & 14" 870 barrels that are legal, but you can't cut down an existing barrel shorter than 18 1/2". Any manufactured TC barrel that installs on your gun and doesn't make the overall length less than 26" and you're good.
My question is if you install a pistol grip on a TC with a long barrel that keeps the overall length over 26", does this make it a pistol since you can put pistol grips on shotguns over 26" and they still remain unrestricted why would doing the same not work for a TC.
Our laws are so conveluded and retarded it's hard to imagine they were made by adults.