Looking for recommendations for proper ammo

[B. Dudley]

Not exactly like “new” since Parkers were never all blued like that.
Whoever did the work did not know how vintage doubles are to be finished.

It looks like the gun has been hot blued which may have compromised the integrity of the barrels. You should get it looked at by someone who knows better to bless the gun as sound.

[Researcher]

When your Parker was made in 1903, the "standard" 20-gauge shell here in North America was a 2 1/2 inch case loaded with 2 to 2 1/4 drams of bulk smokeless powder (or the equivalent in dense smokeless powder) pushing 3/4 or 7/8 ounce of shot. Longer 20-gauge shells of 2 3/4", 2 7/8" or 3" with a bit hotter load of 2 1/2 drams pushing the same 7/8 ounce load were available, but their real or perceived advantage was more/better wadding.

Parker Bros. had a policy of holding their chambers 1/8 inch shorter than the intended shell on the theory that in the days of paper shells and fiber wads the mouth of the case upon firing extending a bit into the forcing cone gave a better gas seal and hence better patterns.

All this to say your chambers, if they haven't been lengthened, may be as short as 2 3/8 inches.