I assumed that with an online name of: homemadeDR, that the proposed build was a DR - not a singleshot. Remove the need to regulate 2 barrels to a specific load and my comments don't really apply - I have single shots in 25-35, 30-40, 45-70, 45-90, 50-110 (all in strong actions) and can tailor loads from sqibb lead bullet to full-house and adjust sights to match. Add the need to regulate 2 barrels and the wide range of loads disappears, and the "rimmed deer cartridge?" criteria prevails!

Let me add or agree w/some of opinions voiced, 30-40 Krag for a "traditional" rimmed cartridge operating at moderate pressure, 30 cal. barrels and reamers readily available and reasonably priced. If factory ammo is a major concern 30-30 is #1, if a double rifle, regulate for 170gr!

If you want something exotic or just different, nothing against 303 Brit. or any of the 6.5mm, 7mm or 8mm (if you were in Canada or Australia or Europe), and 348 Win or 38-55 both certainly capable - but all these suffer from poorer availability of components, loaded ammo, etc. I just spent a couple months debating the same decisions and finally almost bought a Pedersoli Kodiak in 8x57JRS at auction instead of building my first DR, debated calibers based on ready and common availability of gun-making components and factory ammo and reloading stuff. I would still love a 38-55 or 50-110, but out of practicality am half way into my own 30-40 build w/ 2 new barrels profiled and chambered to 30-40. Perhaps 30-30 chambering might've been a better choice, but I personally like 30-40 and have been loading it for 40 years. I had bought the 2 profiled 30 cal blanks for $70- ea., and fellow forum member and DR builder Judson agreed to ream chambers for me for $150-, so I now have under $300- in my barrels and expect to complete this first double rifle with around a $1,000- investment - if I had to rethink it I might've gone with 30-30 instead, awful lot of deer and black bear and moose even have been killed awfully dead w/30-30 over the past century!

Good Luck with your project, Joe(hobbyguymaine)