Sorry, just got back to this. Wonderin if I helped Swamprabbit at all.
Well, you can try that if you want, Ed, but knowing my luck the measurememnts would be changed upon estraction. You will essentialy jam the bullet into the rifling so when you extract it, the rifling could have a tendency to "pull" the bullet, thereby skewing your results. Best to have the tool and just find where it touches, I think.