Re: load choice for 2 1/2" chambers, assuming adequate wall thickness
Prior to 1924, the Belgian and British Service (using) maximum load for 12g 2 1/2 and 2 5/8 shells was 1 1/4 oz. / 3 1/4 Dram Eq. (1220 fps). The pressure of that load would have been about 8500 psi with BULK smokeless powder; with DENSE smokeless 9,500 - 10,500 psi.
2 1/2 shells were usually loaded with 1 1/8 oz. shot and 3 Dr. Eq. of BULK Smokeless with a pressure of 6500 - 7500 psi; DENSE Smokeless was 9000 - 10,000 psi.
During WWI the standard English 12g load was dropped by law (to conserve the supplies of lead and powder) to 1 oz. and 3 Dr. Eq. Bulk smokeless. After the War, 2 1/2 shells were generally loaded with 1 1/16 oz. shot and 3 Dr. Eq. Bulk or Dense smokeless powder.
In the 1925 British Proof House revisions, the 2 1/2 & 2 5/8 12g service load was reduced to 3 Drams with 1 1/8 oz. shot with a mean service pressure of 3 1/4 tons = (converted using Burrard's forumula) 9,682 psi.
