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 Yup that should be right on the money. I just measured mine @ 20" and its about .692" but then I have a short shank on this one, its 5/8" inch so I'm going to be a bit skinnier at 20".
My Lilja #3 @ 20" is about .718" or so - also with a shorter then usual shank.
Not a great picture but here is a Classic with a #3 Brux on the left and a Classic with a Rem sporter on right. The Brux is a perfect fit IMHO.