I needed to know the chamber neck diameter of my Savage 300 model 99. (It's a long boring story, so I'll skip that.) Anyway, maybe this has been figured out already, but here goes. Insert a wooden dowel rod into an empty case. Cut it about 1/4" long. Then drop a lead round ball big enough to work, into the chamber. Then simply load the case and begin closing the bolt. Give it a good whack on the lever when contact is made, then remove slug in the usual manner from the muzzle.
By the way, my 1940 99 neck chamber is .345