Likely you have a gun with smaller bore. Many do. Rim thickness is also variable in this caliber.
I am lucky with my find. All seems to match up. I have a 16 ga and the chambers were already worked with lenghtened forcing cones to allow 2 3/4 inch. I only shoot light pressure loads. Mine is a hammer gun, but has post 1939 proofs. Odd. I don't know if it was reproofed or had new barrels.
My 16x16x9.3x72.