Forgot to mention, I owned back in the early 70's a mint, cased Holland and Holland underlever hammer double rifle steel barreled with 26" barrels that weighed 16 1/2 lbs and was made for the Nawab of Bhopal. It was chambered for a 4 1/4" brass case and the original charge card in the oak and leather case says the charge was 10 DRAMS! Contacting Holland and Holland, they said only a double and a single 4 1/4" 10 dram 12 Bore were ever made. Couldn't afford to keep it, damn it, and sold it to Gary Herman.
Blackhorse, I would not shoot 9 drams out of that rifle. I agree with PM that is probably a round ball rifle with the slow twist and a 5 to 6 dram charge. Maybe I am wrong, but the ball goes down in the case and the case is crimped somewhat over the ball.
