Peter - good idea Peter - blck powder, although producing more recoil with any given 'drams' equivelant load, will also produce less pressure. I failed to note what bullet weight the gun was nitro-proofed for - isn't than normal to have? 1 1/8OZ is only a 14 bore ball weight at 492gr. - abotu .690"

To get to the bottom of the grooves, might demand a patched ball (not a bad idea) as discussed in a different thread. A .715" RB would work, as would a .725" ball, but both would be 1 1/4oz or heavier. My .725" Tanner mould cases .723" in WW metal and they weigh in the 550gr.range, similar to a pure lead .715" ball. To go to a .735" ball will increase the weight to 600gr. A ball at groove diameter would be .740" (620 in WW) or slightly heavier.

That this rifle is Nitro Proofed for 1 1/8oz shot means modern nitro shotshells which produce 11,500LUP/PSI? to 12,000 LUP/PSI? Anyway, by this, you could easily load it with enough black powder and a 550gr. round ball to get 1,600fps easily - probably 6 1/4 drams for this rifle. BTW - that load will kick a bit, but it certainly allows a wide range of loads for you to find what it regulates with. Starting with 3 to 3 1/2 drams is a good idea.

Developing loads for that gun is going to be fun!