Hi- a .645" groove diameter will put it 1/2 way between a 17 and 18 bore. No longer a 20 bore gun.

I suspect a chamber for 20 bore brass might be right, though.

A .645" pure lead ball should weigh around 400gr. even.

17 bore weighs 411.8 and an 18 bore ball runs 388.9gr.

In the 20 bore brass hull - anneal, of course - 2 3/4" or 2 1/2"?

Whatever primer you want, probably brass from Buffalo Arms, probably will use large pistol primers.

2 1/2 to 3 drams of 2F- necessary wads to get the height to the ball and slight crimp- or if tight fit in case, no crimp likely needed. Some of the BP Cowboy action guys seal with water glass & use no crimp on brass. I used a crimp for my ball and patched ball loads, as well as for shot loads in my straight rifled Husqvarna SxS. I made a cone-shaped die for crimping these. These are 16 bore brass and had to be annealed before loading.