The LYMAN SHOTSHELL handbook also has ball, slug and bullet loads listed in it's buckshot and slug data section for 20 as well as 12 bore. I am referring to the older #2 edition, but perhaps the new ones have even better, up to date data.
: Since you guys are talking about the H&H slugers, and that they have them in 20 bore, this data may be of use. The slug they used was the Lyman 340gr., but since it's hollow based as any Foster, it will lack penetration on heavy game. They also list data for the 410gr. and 425gr. Lyman .54 cal slugs. These are loaded inside plastic shot wads, with the wad falling off at the muzzle. This would allow using them as-cast, no lube whatsoever, with no leading, either. Some plastic cases allow loading these up to just over 1,400fps - that's power and penetration for moose and elk, from a little 20 bore - imagine. Unforunately, their round ball data is for choked guns, therefore they use only a .575" round ball. For cylinder or rifled bores, you could use .600", 610" or .615" and the gas seal cup-up to centre and spin the ball in the rifling or in a cylinder smoothbore. If you want some starting places for loads for 20 bore, let me know. The 12 bore thread has some 12 bore loads already.