Wick Ellerbe said:BATF would also frown on that, unless you registered it with them.
Not really. First of all, there is no Federal "registration" requirement. You can build your own cartridge handgun for your own personal use without "registering" it with anyone. What you can't do is build it and then either sell or give it to anyone without first acquiring a tax license.
It is legal to buy a cap and ball revolver, then buy a cartridge conversion cylinder and install and shoot it. You cannot sell the modified gun, however. You can sell the gun with the original c&b cylinder installed and then sell the cylinder separately. They cannot be sent to the same buyer in the same package, however.