Not to you in particular, KEB, but Nominal dimensions Relevant to 20 Gauge barrels are close, as follows:
19 gauge - .629"
20 gauge - .615"( 62 cal.)
21 gauge - .605"
22 gauge - .596"
23 gauge - .587"
24 gauge - .579"( .58 cal.)
Because of the wide range of measurements of both Actual bores sizes, and the size of balls being use, its important to keep in mind that which size ball used, which thickness of patch to choose, or even whether to shoot "bare" balls in your smoothbores, depends on what that Actual bore size IS.
Because you are basically shooting pumpkins( balls), as there is no rifling to put a spin to the ball, some people have successfully used .58 caliber round balls with double patches in their .62 cal. smooth bores. Gas seal is not good; and yardage needs to be shortened. But, they work at short yardage to take deer when you don't have a choice.
The heavy weight of these large bore Lead Balls insures adequate penetration, even at lower velocities. Most Whitetail deer are killed at distances well under 50 yards, even using modern shotguns, rifles, and handguns. :hatsoff: