You can have a Contender or Encore rifle/carbine made in any caliber you like. So it all depends on how well you sell your story. Chances are the pimple faced kid at Sports Authority won't know what you're talking about, be it a lie or the truth.
I think the best answer is to own one of ever caliber. :D
I've had a couple Thompson Centers over the years, and a couple dozen barrels. I never worried about it.
I've registered two Contenders and an Encore and not once did I bring a barrel in with it.
The first time the officer in charge told the person doing the paper work "Put down .35 caliber. They almost always seem to be .35 caliber." (at the time .35 cal or better was required for big game hunting)
I don't recall the second time, I may have just said "35" again.
The third time was for an Encore frame and I was feeling froggy. They asked what caliber and I said there wasn't one. "Well what's it GOING to be?" they said. I told them I didn't know. That I bought it cause it was so cheap, and that I may never buy a barrel for it. Just sell it in the future. They didn't seem to know what to do. This went on for a few minutes.
They finally said "Well I have to out SOMETHING down, is there anything you think you MIGHT want to have one day?" lol I figured I'd pushed my luck long enough and just said ".22".
I'm always vague, just give the caliber. My Ruger Buckeye is listed as ".40" It has two cylinders, 38-40 and 10mm.
There's no point trying to explain to them my .22 Long Snapper Ruger Bisley. (a 22 center fire made from a shortened 22 Hornet) It just says "Ruger .22"