Ruger Blackhawk in .30 Carbine, mounting a scope forty years ago. There was not the mounts nor even the scopes available then that we have now. The answer was a Davis long eye relief adaptor on a Weaver K4, this cut the power to 2X. Unertl bases on the barrel and .22 rimfire Weaver bases. We started with one base and one scope ring, but the .30 Carbine recoil moved the scope. We went to two bases and two rings.
The problem is getting enough purchase on the base to keep the scope from sliding under recoil. Eventually, we drilled through the ring bottom half and the base, installed a drill rod just high enough to allow the rings to come off and which protruded enough to stop the ring movement. Lots of experimentation, but success. In the next five years, there came bases and scopes for handguns.
Five years ago, wanted to switch to internal adjustment scope on my CPA Stevens 44 1/2. Remembering the Ruger .30 Carbine adventure, I started a new search. CPA offers a Weaver base which can be added, but this required drilling and tapping another hole. Weaver bases can be bought with hole dimensions the same as Unertl bases, but you are switching bases out for each change and that is time consuming.
Frank Russell offers an adaptor that slides over the Unertl-style bases and clamps on them. There may be other products similar in function. To switch barrels, requires removing the scope, removing the base and then switching barrels. Then adding back the adaptor and the scope.This adaptor returns the scope close enough to be on paper after a barrel swap.
With the earlier .30 Carbine experience, I do not trust a single ring on the scope. And I like keeping the Unertl bases on for shooting ASSRA and ISSA Traditional matches.
Farm boy from Western Illinois, living in the Magical Pacific Northwest