Expander Mandrel Size?

[centerfire]

Stop measuring OD, you're adding in the neck thickness variation of the brass to a measurement that shouldn't have it. Turn you calipers over and measure the ID of the brass. Then measure the OD of the bullet and subtract the two. Properly annealed brass only needs .003 of neck interference. I use a .262 mandrel and get .003 interference. Rock hard brass will have more spring back and may yield greater interference from the same mandrel (.005 for example).