The smallest caliber/physical size of any current DR is the Heym 80B in 22 Hornet. I have one on order, about a 12 month wait. They size all of their DRs to fit the caliber requested and will build from simple (a bit of scroll engraving) to as elaborate as you want. The result is a tiny but lovely DR that can be -- as will mine -- be scoped and perfectly accurate out to the range of the cartridge. A 'varmint DR.'

I also just acquired a Merkel 141 in 7X57R. Merkel 141s (and the sidelock iteration, the 161) are built on 28ga sized frames and are significantly smaller and lighter than their full sized half brothers, the 140 and 160. I believe the 141 in 7X57R is the current 'champ' for small caliber DRs. The 141s (I also have a 161 ex EWA gun in 7.65R) are an absolute delight to shoot and can be had off the shelf in this caliber or the 7.65R, can be ordered with a scope, and provide low priced, not low quality, DR fun in a chambering that is low recoil, can be 'fed' cheap and off the shelf, and make for fine shooting at reasonable range.

My 'baby' vintage DR is a WR boxlock 22HP. A complete hoot to shoot, nicely made and nicely engraved and accurate. Again, you can buy factory ammo off the shelf for a very reasonable price for those times when you don't have time to handload and, to be frank, the cost savings in this caliber and others such as 7X57R is not as significant as when shooting the bigger Nitro chamberings. I also have a lovely Webley-Scott in 250-3000 I bought from George Caswell last year. Again, zero recoil, easy and cheap to feed by stopping in at the local Gander Mountain on the way to the range and built to size. There is just something very neat and right about a small proportioned DR.

Viva smaller bore DRs Dave