My pacnor barrels are 1 to 20-24, but up to about 440gr for me
in sabot they seem to stabilize in NEF with 1 to 35".
Speed is the difference as RPMs are faster so they
are still stable. I'm sure they'd wobble if I shot them
at only 1200 or so. As far as the real slow twist barrels
look up the guys specializing in black powder guns,
and also check with Dixie Gunworks.

I glued up 3 of 12ga prototype sabots. At 25yds( the 50yd range
is in the water and snow) And the three with 437gr .512"
slugs I did one 2" group with peep sights and bad eyes.
Running about 2400 fps from RMC case in the NEF 12GA FH
Shot 3 of my 28ga FH with Lyman 360gr at 2100, from
the Enfield 28GA FH, and got same size group.Real windy
when testing . Going cold and ready to snow again.
Shot a 600 grain Dixie hardened heatreated slug, in Savage
in our long case at 2900 though two-- 1/4" steel plates with
2" of wood between them. Made big hole.Ed