Looking pretty good.
Also looks like it has a nice crown - bit difficult to see, but looks smooth. A smoothly angled crown will allow a wide range of patch thicknesses to be used. the smooth crown allows the patch to smoothly fold and wrap around the ball as it's seated into the muzzle.
First thing to do, is to measure the bore. Calipers with small tines will be close enough. Find out what size it is, whether it's .615", or a true .620". No matter what size it is, you can use a ball form .020" to .030" smaller than the bore, quite easily.
A .620" bore can use a .600" ball with about a .017" to .020" patch and shoot well, or a slightly smaller ball at .595" might shot fine with the same patch. A tighter .615" bore might be best with a .590" ball. My 20, has a slightly choked (American choke) muzzle. The bore itself is .620", with about a .615" muzzle so I shoot a .595" ball with a .0215" to .022" denim patch, measured with a mic. Measured with 2 different makes of calipers, it goes .024" and .028". I never have to wipe the bore while shooting and the last shot of the day has the same accuracy as the first - loading is unchanged as there is no fouling buildup.
Use a patch thickness that will hold enough lube to keep the fouling soft, so when you load the next one, it cleans the shot just fired. That way, you will never have to clean the bore while shooting.
Some guys use light charges, like 65gr. 2F, which is 2 3/8's drams. This is a good, normal load for use with 7/8oz. shot in a 20 bore, while 75gr. (2 3/4drams)might be a normal load for use with 1oz or even 1 1/8oz. shot. They will also work with round balls, however I've yet to see a 20 bore smoothie that didn't shoot a lot better with at least 3 drams or more using round balls. That is 82gr. or more. 27.3gr. = 1 dram.
I would set up a target at 30 yards or 35 yards and start around 70 or 75gr. 2F, and shoot groups. Since you have 2 sights, that should be easy. You are looking for 1 hole accuracy - 1 to 1 1/8" groups for 5 shots - off the bags. Hold the gun the same as shooting offhand, but resting the back of your left hand (right hand shooter) on a padded rest- sandbags if you have them. Hold the gun tightly, as if shooting offhand and it will give you virtually identical point of impact as if shooting offhand. This works for EVERY gun I have, but it must be held tightly.
Adjust your powder charge up in 5 gr. increments. If round ball use is for hunting deer or black bear mainly, I'd start at 80gr. and go from there until I got great accuracy (for a smooth rifle) - for moose, I'd start at the same and perhaps run as high as about 120gr. I am interested in what sort of accuracy I can get at 50 to 75 yards, and if shooting tightly enough, might try it out to 100 yards. Restrict your 'hunting' range to that which you can hold 10" or tighter.
Note that with a smoothbore, the heavier the charge, the farther out the ball gets before doing it's trumpet-end, flight syndrome spinning off somewhere other than where it's pointed.
We have a few guys who shoot light charges in their 20 bores. I've watched these low velocity balls spinning off to impact 1" to 3" off the longer ranged targets, 90 yards or farther. Binoculars show this quite vividly, in the right light. Other times, you can't see the ball's flight. Sometimes they get straight flight, but a lot of drop, then another goes spinning off well away from the target, just like a curve ball thown by a baseball pitcher. With heavier charges, the balls are seen to fly quite straight with much less drop.
We use short starters for seating the ball into the muzzle. If you want to shoot a weaker combination that you can push into the muzzle with your finger, your accuracy will probably suffer, and the bore will foul. Those are my findings.
