Recently I started a comparison of Goex 2F and 3F and roundballs in a .54 Green River Hawken, using published charges and chronographing it to compare to them. Thought I might as well compare the accuracy at the same time, but can't aim precisely enough with that buckhorn sight. So I mounted a scope. It is a Weaver 4X. I fitted a block of wood between the rear sight and the breech snail, inletting the octagon barrel into it and letting it rest on the stock. It had to be high enough that when I inletted the scope into it, behind the rear sight, the scope objective lens cleared the barrel and the hammer cleared the scope. The block was about an inch high. I clamped the rifle in a vise and aimed it at a target and lined the crosshairs up so it aimed to the same place. I used hose clamps from clothes drier vents to hold it, padding the stock with leather. You could use duct tape to protect the scope from the pan flash. This works well, but I got side tracked by driving to Chicago to hear a friend from Scotland play guitar and sing in an Irish pub, then went to Minneapolis for a conference and back through Valentine, NE and the Museum of the Fur Trade at Chadron, NE and Cheyenne. Will get back to this and will post a photo eventually of the test results.
I'd recommend using a staight 3/4" tube .22 scope. I think Wal Mart sells one. You don't need more than 4X magnification, and the smaller tube would be easier to mount.