I have an H&H dominion 16 that has 1oz nitro proofs. Because pressure is pressure, however, would it be possible to shoot lower pressure (e.g., 8k psi) 1 1/8oz reloads in it without ill effects? The wood is solid.

Sporting Guns and Gunpowders: Comprising a Selection from Reports of Experiments, and Other Articles Published in the "Field" Newspaper, Relative to Firearms and Explosives, Volumes 1-2, 1897
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA296&dq
Pressures were measured using crushers (LUP) and reported in pounds/ sq. inch; modern piezoelectric transducer pressures would be 10 - 14% higher.

Major Sir Gerald Burrard, The Modern Shotgun, Volume II, “The Cartridge”, 1955 3rd Revised Edition pressures converted from Long Tons/ Sq. Inch (no 10-14% addition)
................................Standard service pressure - Maximum service pressures
16g 2 1/2” 7/8 oz. 2 1/2 Dr. Eq. - 8,624 psi - 11,312 psi
16g 2 3/4” 1 oz. 2 7/8 Dr. Eq. - 9,296 psi - 11,984 psi

Thank you Drew, that's very helpful.

More numbers very similar to today's 16g loads for you Mark

William Bruette, Guncraft: Guns, Ammunition, Wing & Trap Shooting, 1912
https://books.google.com/books?id=5g51K93as84C&pg=PA210&dq
Pressures converted from long tons/sq. inch to psi by Burrard’s formula - no need to add 10-14%
Curtis & Harvey’s (42 gr. = 3 Dram) Amberite (Bulk)
16g 2 1/2” case 36 gr. (2 1/2 Dram) 15/16 oz. = 10,640 psi
Curtis & Harvey’s Smokeless Diamond (33 gr. = 3 Dram)
16g 2 1/2” case 28 gr. (2 1/2 Dram) 15/16 oz. = 11,480 psi (SAAMI max. is 11,500 psi)

Why don't you just stick with the load it was designed and proofed for? Why do you need the extra 1/8 ounce? It seems to me that it is just not that important. If you want to shoot 1-1/8 oz loads then get a 12 bore. At any rate, the best load for a 16 bore is 1 ounce. It will pattern better and be a little easier on the gun. I have found a 1 oz load in a 16 gauge double to be one of the most lethal loads around. It has a very short shot string and hits hard.

35-50 yd shots on chukars going away. 1oz gets a bit thin.

My “go to” load for upland game, including chukars in the Snake River country is 1 ounce of #7 shot. Never have felt handicapped with it. I use it on pheasants, huns,grouse, and both bobwhite and blue quail.