Perfectly legal. The issue is usually conversion cost. Could exceed the revolver's value. What is it chambered for now? What rimfire caliber do you want to change to? Is it a black powder gun? If it is, converting it will still leave it as black powder, meaning no similar-caliber modern smokeless ammo at standard pressure.
I keep forgetting to take a photo of mine, but here's an identical one. I know it's not very valuable, but I love it and would want to shoot it often, so to me, depending on the cost, could be worth the conversion to me.
Anyone know a gunsmith that does this kind of work?
I know the expression is "anything is possable" , but this situation is straining it.
It's doubtful there are any living 'smiths on this continent with experience in extensive custon work on solid frame Webleys . ( And that's even assuming the gun in question actually IS a Webley , and not a ubitiquos Belgin knockoff.)
If it's what I'm thinking , there weren't any origional rimfires to canabilive for parts , so key components would have to be designed , and made from raw stock.
If you could find anyone to take this on , it would run into the price range of a decent used car.
For less than that you could get a lifetime supply of ammo for origional cal , plus several modern-ish rimfires.