I know James of Dixie, he's a great guy.
With your NEF 20ga, If you can get a good measurement of the groove dimension, this will help. Solid slugs give less pressure than that of a shot column. The reason is the shot acts as stemming and "wedges" against the bore. On a solid slug it simply is pushed down the barrel and the only "tight" spot is the engraving of the rifling. Once that is done it is simply a movable plug.
I have found that +.003 for the 20 bore is quite acceptable, and will aide in getting better accuracy. Where the shotguns really falter is with the forcing cone being cut for shot columns, even on slug guns. This leaves the bullet unsupported in is transfer from case to barrel, and many times is far oversize to your slug. It allows the slug to cant and hit the rifling unsquare. Fatter bullets will aide in this as will using 3" hulls matching the chamber length.
Let me know how you get along.