Can't ever remember seeing 20 gauge.. You can go to the Remington, Federal, and Winchester web site and do a inquiry.
Me I reload my own 12 gauge BB and # 4 buckshot so you can do the same. I use the MEC press to De-prime, Prime, reload the powder. Put in a wad.. Then I add the shot with out the press. 12 # 4 buck has about 27 pellets..BB I weight what 1 1/4 ounce of shot on my reloading scale. They I pour it into the shotgun shell. The shell goes back to the MEC press for crimping...Takes a little longer. But I do have BB and # 4 buckshot cheap... I use the Nickel Plated lead shot.. Ballistic Products is your source of shot, wads, etc. The IMR, Winchester reloading site is your source for the reloading data...