Here are the two tools we keep referring to.
The one on the left is a Lyman case gauge.
The tool on the right is the Hornady headspace gauge body with a Sinclair headspace insert and the Hornady anvil all attached to a caliper.
The case gauge is quicker to use but it isn't as precise, plus it's a gauge, not a measuring device. All the case gauge does is tell you if the case passes or fails the minimum standards, it wont tell you how much it passes or fails by. The headspace gauge will give you numbers, they might be relative rather than true values (if the insert is worn or the caliper is out of calibration) but the numbers can be used to compare one case against another.
Here is an example of how to use both.
A case that started out measuring 2.026" long from the base to the mouth also measured 1.632" at the headspace datum line on the shoulder.
It looked like this in the case gauge. The black thing on top of the case gauge is a flat spot on my caliper that I used as a flat bar that I placed over the base of the case.
Headspace
Notice the slight gap between the flat bar and the top of the gauge (on either side of the case).
Normally you can't fit an unsized case into the gauge but in this circumstance, I used a case that had been resized too long and hadn't been trimmed yet.
Trimmed-To-Length
Here is the other end of the case gauge and you can see that the mouth of the case extends past the top of the gauge. This case is 0.011" too long.
The case gauge let you know that the case was too large for the chamber, it just didn't tell you by how much. The Hornady headspace gauge, on the other hand, told me exactly what the dimensions were.
So I resized and trimmed the case.
Headspace
Now 1.629"
Which looks like this in the Lyman gauge. Notice that now there is no gap between the flat bar and the top of the gauge.
Trimmed-To-Length
I trimmed the case to the SAAMI max, 2.015" just so you could see how it looks in the case gauge.
Here is the caliper reading.
And here is the Lyman case gauge.
I hope this wasn't too long, but it seemed like we had enough new guys that I thought it might be beneficial to show them what we are talking about in detail.