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Untrue. you are only measuring the case at it's mouth
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If the mouth is wider than the rest of the casing, a caliper (which only non-machinists think of as a precise tool) will find two good points to rest on. If it's not, you can get a deceptively large value because your calipers will rest somewhere behind the mouth.
SAAMI says the rear of a .45 ACP cartridge is wider than the mouth.
I think I should have used the caliper anyway, because even though it's sloppy, it seems to work fine for other people. As a machinist, I can't get used to the fact that people really try to take precise measurements with dial calipers. No machinist does that.