Hello:
While I have not attempted to re-bore a H&H I have had great success with another large caliber rifle. The bore of the rifle had poor rifling from poor storage, the gun itself was nearly pristine. I contacted a number of 'Experts" and we found that by re-boring we may be able to get the original caliber back, once the heavy barrel was re-bored, it was then sleeved, the sleeve was rifled into the original caliber. The only way one could really notice was the muzzle, it appeared to have a sleeve, and it did and it functioned great. Now, I just purchased a very nice Cape gun, 16 Gauge and a rifle caliber that is to obsolete to find brass for, I intend to do the same with it. Double guns are different and not just barrels resting S/S, the gunsmith knows this or should, I am sure my Cape will turn out just as good as the rifle. I hope this helps....barrels can be sleeved-bored-etc. There really is a couple quetions though, Collector value and the shooting condition, both should be reviewed before attempting any modifications, I am a shooter as well as a collector, I shoot what I collect. Cheers.
Michael