In the time it takes to dissolve the lead, mercury will also plate the inside of the barrel and then every time the rifle is shot, highly toxic mercury vapour will also shoot out. Given the value of the rifle, I'd say don't use mercury.
I remember many years ago I was in a Friends basement looking at his collection of things and he was an avid black powder shooter . I noticed a glass bottle with a glass stopper in it up in the rafters. I said what is that . it was a bottle full of mercury . he said he used it to clean his barrels . I mentioned that to someone and they said you get toxic fumes off it . I haven't spoken to him in years and heard he had passed away so I think I'll call his wife and mention it to her . she's a good woman .