To make a 14 or 12 bore would require a straight or straight taper 1 1/8" to 1" or very small swamp from a larger, say, 1 1/4" breech due to the more emtal removed. The gun's weight is important or it will kick to much to be enjoyable shooting. If it's enjoyable to shoot, one will learn the rifle and be able to hit well with it, otherwise, it's usless as a hunting rifle.

1 1/8" tapered to 1" at the muzzle would be OK in a 32" bl. in 16 to 14. The barrel would be quite light (too light?) in 12 bore as it would barely make 9 pounds in 14 bore. Due to the powder charges used, one might want to target 9 1/2 pounds. My 14 bore, pictured below, has a straight taper 1 1/8" bl. 32" long. The rifle weighs 9 pounds 7 ounces exactly (unloaded - 9 pounds 9 pounds 8.4 ounces loaded) (edited weight)

Here's the recoil of a normal hunting load in my own .69 (14 bore) rifle, shooting a 482gr. pure lead round ball with 140gr. 2F. I used to use 165gr. for hunting, but we were shooting a 200yard RB contest and 140 shot to the sights at that range. 165gr. zero'd at 200 meters so was 4-6" high at 200yards.

Sorry for those who are sick, of seeing these pictures, caught before and at the peak of recoil's rise. The rifle is a 14 bore, charge 140gr. 2f GOEX, 482gr. Round ball.