[Drew Hause]

My apology for the caveat, but no one can tell you if any load is safe in YOUR gun.

Feb 24, 1900 Sporting Life
The Winchester people offer the Repeater in 10, 12, 16 and 20 gauges, loaded with any of the standard brand of nitro powders; the maximum loads for this shell being 10-gauge, 3 1/2 drams powder, 1 1/4 oz. shot (shell boxes list 28 grains Ballistite or Laflin & Rand Bulk Powder).
We know 12g 1 1/4 oz. 3 1/2 Dr. Eq. BULK Smokeless was about 11,750 psi; DENSE Smokeless was 12,600 psi. 10g would certainly be less.

Until the 1920s, the heaviest North American factory loaded 10 gauge shells offered were 1 1/4 ounces of shot with 4 1/4 Dr. Eq. of smokeless powder in a 2 7/8 inch case. The Western Cartridge Co. Super-X load Super-Ten shell with 1 5/8 ounces of shot with 4 3/4 Dr. Eq. of Progressive Burning Smokeless Powder in a 2 7/8 inch case was introduced about 1926.

A Hunter Arms Co. Pressure Curve dated June 10, 1929, from the McCracken Research Library, Buffalo Bill Center of the West lists 12g 3” ‘Record’ 1 3/8 oz. shot (presumed 1275 – 1295 fps) at 13,250 psi + 10-14% for modern piezo transducer numbers. No 10g service load was included.

Those loads very likely are at the SAAMI recommended 2 7/8" maximum of 11,000 psi, and recoil would be substantial.

Recoil in a 12g 1 5/8 oz. 4 Dr. Eq. (1280 fps) in a 9# gun = 40 ft/lbs.
1 1/8 oz. at 1200, (3 Dram) in a 7.5# shotgun = 23.0 ft/lbs