The only way to know is to go back to TC and see what it was when it left the factory or figure out who originally bought it and ask them. If it was a rifle it'd need to be registered, if it was a pistol you're fine, if it was a bare receiver it would be difficult at best to prove it was built into a rifle before a pistol. I'm not sure if TC will respond to an individual query, but they'd have to respond to a trace initiated by law enforcement if it ever went that far. You're probably in a real bad place already of that happens.
You could do a 16" barrel and a crusher loop that gets you over 26" OAL if it started as a rifle to avoid registering it. SBRing it also means engraving it and Form 20s for interstate travel.
To answer your original question, I like 15" barrels. I have a few that are a little longer to get a little more velocity, but most are 15". Shorter (10" or so) would be my preference for hunting from a stand, but I use a revolver instead of a single shot in those cases.