I would look for a jones under lever with bad barrels, they can often be had for very low dollar. Your idea of building on a larger gauge frame than the rifle gauge you intend to build in is a very good one. Look in the Greener for finished rifle weights for the gun you plan to build. You want it to finish quite a bit heavier than a shotgun in the same gauge but not so heavy it does not balance well.

I have way too many projects on hand currently but I have wanted to build a bore rifle for some time now. I picked up a gun out of auction in the UK that started life as a .58 caliber muzzleloading stalking rifle and when I bought it had been converted to a jones underlever shotgun and had a bad barrel. I will eventually re-barrel that one for myself, likely in a short version of a .577 or .500 3".

Steve