Histrory buff:
First, you have to clean the oil and grease out of the gun before you go to the range. Use alcohol to dissolve the oil and grease, and dry the barrel thoroughly.
Second, it sound like your first loading did not create enough compression for efficient burning. That happened to me 2 years ago when I got my new Fowler. In my case, the barrel was not 20 gauge, but closer to 19 gauge! The wads I had bought were too small to work. And that .600 round ball was just about too small for the gun.
I am using 70 grains( 2 3/4 drams) of 2F powder as my load.( The ball weighs 7/8 oz.) I put a card wad( 19 ga) down on the powder. I then use a 20 ga. cushion wad lubed with moosemilk, and squeezed out, but any liquid cooking oil will work, as suggested already above, and then run the patched ball down on top of the cushion wad. If I was loading shot instead of a round ball, I would run a over- shot card wad on top of the cushion wad, to keep the base of the shot column square to the bore so it exits the barrel at the same time. After I run the patched round ball down on the cushion wad, I lube up a cleaning patch, and run that down the barrel on top of the ball, and back out again. We found from using our chronograph that lubing the barrel helps makes the velocity of each shot about the same, with a lower Standard Deviation in Velocity, to use modern techno-speak, than we got using the same loads, but without lubing the barrel. Got the idea from some fine shooters in Rockford, Illinois who lube their chunk guns that way. Tried it in our rifles, and then for the heck of it, in the fowlers, and found it actually worked in smootbores, too !!
If you don't use the proper wads, you will lose compression, and velocity, and will have problems with incomplete burning, as you now experience. You can use two 1/8 card wads, in the field, for the load I give above, and skip the cushion wad. The velocity, BTW, is at 1050 fps, below the speed of sound. That is just fine, as coming down through the sound barrier does all kinds of nasty things to a round ball, and none of it is good for accuracy. The lack of compression is another reason I think you are having such large groups. The answer is not more powder; it is using card wads under that patched round ball. I would increase patch thickness to .020" and lube with OxYoke 1000 Wonder lube. I have used this for more than 20 years, and have no problems with it. Between shots, you can simply run a brush down the barrel to loosen the crud, and then load the next load. In the field, you won't be shooting that much to worry about how much time it takes to clean between shots. Keep a jag on your wiping stick- I carry mine down my back, held to the body with the strap for my possible bag, and the jag stuck into the small of my back in my jeans- and use a liberal amount of cleaning fluid on a big patch- i use the ones sold for 12 gauge- to wipe out the gunk. Then I run 2 or three dry patches down to dry the barrel completely before loading the next powder charge.
Third: ALWAYS run a vent pick into the vent to make a hole in the main charge so that the flame from your prime is able to reach and ignite multiple granules of powder. The vent pick will also clear that channel you found, keeping the gun one shot dirty at most.
Oh, just so you understand how important using the wads are, my velocity went up 200 fps when I started using the wads ! And standard deviation of velocity dropped to about 20 fps. You can't do better shooting modern shotgun shells !
