Personally, I'm pretty interested in rifling a barrel for some ball-type ammo to stabilize shots, and just for the sake of nerf modding.
This guy does rifled barrels for spud guns, most of the barrels are fairly large though. The smallest size is 1.5", i don't know if that helps.
Here's the url, not sure how to get the tags to work, sorry: http://www.spudtech.com/default.asp
Edit: ok, it sort of worked.