A 30-06 seems attractive because of familiarity, but double rifles should really be fed proper rimmed cartridges. 7x57R, 7x65R, 8x57IRS, 9.3x74R are the small bore double rifle standards. Rimless rounds require finicky and fragile extractor plungers and are more difficult to pull out with gloves on. I personally wouldn't own a double rifle in a rimless chambering. However, I certainly agree with adjustable regulation on a small bore double rifle. I have had two rifles re-regulated, and it cost over $2,000 each time. Worth it to me, but expensive! I'd rather buy a more expensive rifle that has a regulation system built in, and do it myself when necessary.