Dino:
I just relined a 32-20 winchester 1873 so I can tell you a few things. First, most gunsmiths install premade liners made by Redmans and available from Brownell's. The come in the standard cowboy calibers and .30cal. They don't make one in 50cal. This means that you woulod have to buy a 50cal barrel and turn it down to a liner (tough because the barrel will flex in the process because of the small size). Also, to properly line a barrel, read get it to shoot at all, you have to have the proper long piloted drill. The standard liners have standard drills for them (which cost between $60 and $80). You would have to have a custom drill made to fit the liner you have made. All this is to say that while it is possible to have this done, it will cost a mint and no gunsmith will guarantee that the gun will shoot well.
I had a similar problem but I chose to make an entirely new barrel. I've tried to post a picture of my gun but I'm having problems with the download. I'll post it from a different machine tomarrow. I'm particulalrly proud of the front sight lug which is integral to the barrel. I machined a ring the right size then cut away the unneeded parts off with a hacksaw and finished it with a file. This had to be done with the barrel installed so the sight is in the right place. I have to say that this was a bitch of a job that my dad and I did together. He still says that this is the worst job he has ever done (and he used to machine stuff for NASA back in the 60s!)
By the by, I'm in Reno to, I wonder if we have met ? I used to do all the gun shows and was a partner in High Plains Drifter Gun Shop in Sparks.