OK. Why would I want a C&B conversion for a Modern revolver?
If I wanted to spend a lot of time at the open air range for very little money, my rickety old brass frame repro 1851 and 1858 revolvers are a lot of fun still yet. I imagine a modern C&B revolver could be cheap fun too.
I have shot my .45 autorim Webley Mark IV, .357 Security Six and my son's .357 GP100 with BP in the local black powder cartridge matches.*
The Webley has a collar on the front of the cylinder to keep BP residue off the cylinder pin, as do my BP era Smith & Wesson and Forehand & Wadsworth cartridge revolvers.
The modern revolvers do not have a protective collar on the front of the cylinder. Smokeless residue does not gum up the works like BP residue plus bullet lube, so why have a protective collar? Because when you shoot a modern revolver with black powder, the BP residue on the cylinder pin does gum up the works and is very hard to clean.
Bottom line to me is that, for a C&B conversion cylinder on a modern revolver, they just do not handle BP residue on the cylinder pin very well.
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*(The BP match rule is cartridges must be loaded with Black Powder or BP substitute, Pyrodex, 777, etc. The guns can be modern. but most participants go traditional BP cartridge arms. Muzzleloadin rifles and C&B revolvers may be used. Pistol events total 20 shots for score, 7 matches per season, total 140 shots for score not counting load development. I've been at it for well over a decade so early on I fired a lot of BP .45 ACP in moon clip and .38 Special. Cleaning the Security Six and GP100 were a pain and a half; ypu really need to remove the cylinder from the yoke.)