FWIW, most aftermarket barrels taper ~.010 per inch, beginning +/- 6" from the breach end. That said, best guess is the Bartlein #2b will be ~.710" @ 20"; #3, ~.720" @ @ 20". YMMV depending on how much shank you chose to leave on the barrel.

You could always call Tracy @ Bartlein.

Bartlein Contours

I never thought I'd grow up to be a grumpy old man, but I did, and I'm killin' it.