Under the 1896 Rules of Proof Pressures 12g 2 1/2” and 2 5/8” chambers (bore .710-.740) were proved for a maximum service load of 3 1/4 Dram Eq. with 1 1/4 oz. shot. (1220 fps)
The Proof House did not publish pressures but other period published pressures with Bulk Smokeless powder were 7,000 - 8,500 psi

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
Curtis & Harvey’s (42 gr. = 3 Dram) Amberite
12g 2 1/2” case 47 gr. (3 1/3 Dram) 1 1/4 oz. shot = 11,480 psi
12g 2 1/2” case 42 gr. (3 Dram) 1 1/8 oz. = 8,960 psi
Curtis & Harvey’s Smokeless Diamond (33 gr. = 3 Dram)
12g 2 1/2” case 39 gr. (3 1/2 Dram) 1 3/16 oz. = 11,984 psi
12g 2 1/2” case 33 gr. (3 Dram) 1 1/8 oz. = 9,632 psi
12g 2 1/2” case 33 gr. (3 Dram) 1 oz. = 8,288 psi

In 1925, the 2 1/2” & 2 5/8” 12g maximum service load was reduced to 3 Dr. Eq. with 1 1/8 oz. shot with a mean pressure of 3 1/4 tons by LUP = 9,800 psi by Burrard’s conversion.

Bell's study (here about 1/3 down https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit ) showed an increase pressure using 2 3/4" shells in 2 1/2" 12g chambers from 228 to 1216 psi.
His summary statement was "“Shooting 2 3/4” shells in 2 1/2” chambers does make them produce more pressure-but in most cases it is less than a 1000 psi increase. I see no reason, related to safety, to modify an original 2 1/2” chambered gun to shoot 2 3/4” shells, if the 2 3/4” load you intend to use would develop pressure that is safe in that gun, when fired in a standard chamber!”

It should be noted that modern nominal 2 3/4” (after firing) shells may be shorter than 2 3/4”; I've measured some Fiocchi and Federals at 2 11/16".

If the case mouth of a 2 3/4" hull is “feathered”, it very likely extended into the forcing cone on firing with the potential to significantly increase pressure and recoil.