Individual barrel groups with the Lyman slug groups are great!

Now it's time to handload some and tighten up those groups. Lyman's latest shotshell manual has good loads for that slug that should bring the tubes together, as-in loads to 1,550fps or a bit higher. That will handle about anything you need to shoot as that about duplicates the heavier round ball black powder loads for the 12 bore, according to Greener's book.

If you follow Ed Huble's thread, there are loads in there that
will increase your 'power' even more, but more care must be used with Hodgdon's Longshot and AA Steel powders.

Seems to me, Lyman also has some Longshot powder loads, along with Herco, Blue Dog and Unique.