Some of the flare pistols are fairly sturdy. You could likely sleeve it for a smaller caliber, such as a .410, much easier. Anyone with a lathe could make such a conversion. A .410 might be a little easier to shoot one handed, as well.
I think that I would like just about any modern scoped rifle for standoff range. Perhaps a .270 so it is not a “military” caliber. You can get a savage 110 brand new with scope for less than $600. If you shop a bit, you can likely pick up a used one for less than $400.
http://buy.auctionarms.com/10043723/savage-arms-corp-110-gxp3-270-win-with-accu-trigger
A bolt is not best in this role. The absolute KEY is fast followup shots, tracking and correcting off of each splash. This incredible advantage is lost with a slow bolt gun.
And no scope over 4X will work AT ALL due to all of the movement.