Thanks I'll probably initially try 0.820 balls only because I found them easier to buy.

I'm planning to do a proof like test on the barrel and then do a dye penetrant test just to be on the safe side. Following suggestion of another person on another forum I'm planning to use 10% over the service load and 30% over the shot load for this test.I initially planned to build a wooden contraption to hold the barrel and a caplock, but I'm leaning towards doing that test with the barrel in the gun with it suspended on two sets of V shaped pieces of thick rope. This should allow the gun to swing freely and lower the impact of recoil on the stock hopefully. I saw a test like this done in a youtube video after a custom big bore double was made.

However, I discovered another issue with the gun. The nipple is not original (not a surprise in a 150 year old gun), but the nipple hole seems to be taper threaded or the top of the thread has been worn to the point where the top of the hole has diameter of 0.342 ( 8.7mm ) and the bottom is 0.263 (6.7mm). Then the nipple has a metric M8x1 thread which grabs only by 2.5 turns at the bottom of the hole. Although I calculated that in the worst case scenario regarding steel quality every single turn of the thread can withstand 1100 punds of force (5KN) and even 10k psi of pressure produces only 750 pounds of force on the area of the nipple so it should be fine, I'm considering making a new nipple. Making that tapered thread on the lathe without a taper attachment will be quite difficult.

I definitely try the scotch-brite polishing method.