Oh.
Well, then I guess plan b is mic the chokes, and look for a slug that fits your barrels best.
The rest isn't that tough to do.

FWIW, I match bullet/sabot thickness to the barrels on my muzzleloaders to get better accuracy with them.
Sabots are available in .001 increments, I don't know if slugs are. Somebody is probably molding some.

Out there doing it best I can.