I would appreciate any help people can give to identify this chambering in my black powder double, just acquired. I bought the rifle from an owner who had never fired it, and was unsure but thought it was possibly .500/.450. The only calibre marking is 56 in what I take to be the view proofs, (approximately .436?) but that would be before final bore and rifling.

I bought some Cerrosafe and did a chamber cast. It came out fuzzy due to oil I sprayed in as release agent, but gives a good indication of shape. The sketch is dimensioned in mm, and below are the conversions to decimal inch. The measurements were taken with a vernier caliper, and are only plus-minus a thou or two.

mm Inches
25.4
2.3 0.091 Rim
11.75 0.463 groove depth in first inch of rifling
12.55 0.494 Case mouth outside dia
12.75 0.502 ahead of shoulder
13.88 0.546 before shoulder
14.83 0.584 ahead of rim
16.58 0.653 outer dia of rim
69.14 2.722 Case length
52.6 2.071 start of neck
45.7 1.799 start of shoulder

I will have another shot to get a crisp casting later, and borrow a micrometer to get throat diameter more exactly.

The case length is 2.722" max.

So, can anyone identify the round precisely from this? Your help will be greatly appreciated.