Isn't it a Rem 700 footprint? so...would a Rem 700 bore guide fit?
I have a B-14 R and the Possum Hollow boreguide for a Rem. 40x. It will fit, but it only measures .696" There is a little slop in the fit when you insert it into the action. If I had to guess, I would go with an OD on the boreguide of .700. I have been applying a layer of Scotch transparent tape to 1 side if the boreguide ad that measured .699. There was just a slight amount of play when inserted.A bore guide for a Rem. 40x .22 LR might work, though.