Bore diameters are all over the place, and does it really make any difference? Point where the bird/target is going to be and they die/break.
The tightest bores I've measured are on my Fox Gun Co., Balto., MD., U.S.A. doubles. Both are twist barrel A-Grades and both were chambered for the longest 12-gauge shells available in those days, 3 1/4 inchers!! Serial number 153 has a bore of .712" in the right barrel and .708" in the left barrel. Serial number 714 has a bore of .713" in the right barrel and .710" in the left. The biggest 12-gauge bores I've measured are on Parker Bros. doubles from the late 1880s/early 1890s where .750" to .757" are common. I've been shooting Pheasants for 45 seasons with a 1914-vintage Ansley H. Fox A-Grade with bores of .724". I've got two Remington Model 31 TCs and they both sport bores of .744" while my other Model 31s have bores in the .728"-.730" range.
A few years ago many Trap shooters believed the Stan Baker "Big Bore" barrel would get them an extra target or two, and it was a .800" chamber all the way to the choke!
