Did it once with a 54cal. My method was rather crude but, I welded an extension on to a drill bit ,clamped it into a length of angle iron to keep it straight while welding, came out almost perfect. Next started to drill with a 14.4 volt makita battery drill ,take it slow, it won't be easy. Next after drilling, since I did not have any reamers, I got one of those 2 stone brake cylinder hones and attached it to an aluminum arrow shaft and started to hone with the shaft attached to the drill , buy plenty of extra stones and a can of WD40 for lubricating the stones keep it wet while honing. After about 3 days of honing you will wish you never started this project,but keep going the results will be satisfying.
I have taken plenty of squirrel and yes a fall turkey. Mine is also a flintlock.
Have fun.
Not sure if the brake hone will work on a 50cal ,if I recall the hone was plenty tight in a 54cal.