I fully concur with what Bob said about dial calipers. Don't fool yourself into thinking you can eyeball a caliper measurement down to 1/10,000th of an inch as you mentioned above. There is too much slop in a dial caliper to achieve any where near that degree of accuracy.
When you zero your caliper, do you wipe the blades free of any contaminants (use your finger, not a cloth)? Is the dial set precisely to 0 with the blades touching? If it is, and you are still getting .450 for those bullets, I would send the calipers back to Midway(?) for exchange. They should be able to register the difference between .450 and .451 and they appararently aren't.