One of the downsides to those plunk gauges is if you have a case rim anomaly, the case will not plunk all of the way down into the gauge, giving you a false reading.
I've had this happen often, especially cases fired in a gas gun.
No way you're getting .050" difference. Try a different piece of brass.
Years ago, I moved from the plunk gauges to a better datum measuring tool like the Hornady comparator tool. There are several of these type comparators available. My favorite is currently the Derraco tool.
The other tool that measures the datum well is the RCBS precision mic and the Whidden case gauge, both of these gauges are cartridge specific, so it's a little pricey for one cartridge.
The Hornady type of comparator tool works for dozens of cartridges. A caution with this type tool is to decap the brass you're going to measure because primer cratering can give you false readings.