Check the internal bore dimension. YOu may have a barrel that is either narrower, or wider, than the .615 nominal bore diameter, and that will control the performance of your wads, and choice of diameter of ball and patches, if you intend to shoot round ball. Mine turned out to measure .622, substantially closer to 19 ga. that I had to order 19 ga. over powder wads, and over shot cards. I can use the 20 ga. cushion wads, and that actually makes loading faster, than using 19 ga. cushion wads. As to choice of loads, always check your own load over a chronograph. Boorow one or buy one, but use a chronograph. The chrony is more than adequate and sells for under $100.
Bob Spenser's site has loading data from the late V.M. Starr.
[url] http://members.aye.net/~bspen/index.html[/url]
Read those articles, but in general, check the loads now sold for modern 20 ga. shotgun shells. Standard shells, not magnums, or steel shot. The dram equivalent means just that: They are shooting smokeless powder loads that match the loads you would get if you use Black Powder, and measure it by the dram.( 27.5 grains) I think you will find that loads from 2 drams( 55 grains) up to 2 3/4 drams( 76 grains) is the working range for a 20 gauge gun, regardless of barrel length. Starr recommends loads on the lighter side for better patterns. I don't think he recommends a load using more than 1 oz. of shot, and most are less.
Your 20 ga. shotgun will be a 25-30 yd maximum range gun, without choke. There is no reason to stuff either a large amount of powder, or shot down the barrel to hit targets at those ranges, with enough force to kill. Most shooters today tend to overload their guns, which only results in blown patterns, and unpredictable patterns( worse!) I load 2 1/4 dram( 62. grains ) of FFg, then my 19 ga. overpowder wad, then 1/2 a 20 ga. cushion wad lubed in moose milk, then an overshot card, 3/4 ox. shot, and then another overshot card in 19 ga. The overshot card I load on top of the cushion wad keeps shot from imbedding in the cushion wad, while giving a hard smooth surface to push the shot out of the barrel evenly. I use an awl to put a hole in the center of each over shot card to allow air to escape to ease loading.