You have to measure the bores, as you now know. YOU CAN'T GUESS, nor can you rely on any markings on the barrels!
You are NOT the Lone Ranger on this. I have a custom-made fowling piece that was suppose to be 20 gauge( .615" nominally). When I used 20 gauge OP wads and OS cards, they all went down WAY TOO EASILY! I then measured the bore and found it measures .626-.627"- almost a nominal 19 GAUGE( .629")!!! I ordered 19 gauge OP wad and OS cards, and the MV rose some 250 FPS! Talk about a difference in POI( Point of Impact using Round Balls) and patterns!
All wads serve the function of sealing gas behind your load of shot, or holding the load in place until you fire the gun. An OP wad seals the powder from the shot. A " Cushion wad"- that 5/8" thick wad you used normally is lubricated with some grease, or oil, or combination to grease the bore going down, and soften BP carbon fouling in the barrel after its fired.
The Over Shot card holds the load of shot in place inside the barrel. However, the OS card also provides resistance and therefore increases COMPRESSION within the barrel to insure that the powder charge burns completely and uniformly, shot after shot. Its the uniformity of compression shot after shot that creates uniform( Predictable) patterns down range.
If you have a barrel that has muzzle choke, you may find it next to impossible to fit a correctly sized OP wad, or Cushion Wad down the muzzle. It depends on how much choke is in the muzzle.
The answer is to use OS cards instead of the OP wad, and cushion wads. The thin OS cards can be bent to fit thru the choke, then turned and run down the bore. The OS card should easily pivot 90 degrees and go down the bore square to the bore.
Put an off-center hole in each OS card, to let air escape thru the hole, rather than turn an edge and ruin the SEAL! Turn the cards at least 90 degrees, so that you don't turn the same edge where the card pivots, nor align the holes up with one another. That prevents "Dieseling", and speeds up loading in the field.
I carry a bag of greased cleaning patches and run one down the barrel before loading the first shot to be fired out of the gun. After my first shot, I run a greased patch down the bore and out after seating the load of shot and the OS card(s) to hold the load in place. This protects the bore from rusting on wet days and keeps subsequent fouling soft. I also found that greasing the bore protects the shot pellets from rubbing off lead on my bores, which makes cleaning the gun much easier. I don't have to use modern lead solvents to remove lead streaks in the barrel. Without the flats, a few more pellets seem to always end up in my shot patterns, too. :grin: :thumbsup: