You need a way to accuratly measure to a datum point on the shoulder area of the chamber, and as your looking to see if it has increases from what it was you also need to know what it was before.
Take it to a gunsmith, the cost will be less than the price of a gauge (if your a regular customer it might even be free).
BTW, the AI's use a standard "GO" gauge for the NO-GO measurment.

For most bottleneck guns, you can just get a commercial cartridge, and put a small dab of light grease on the shoulder. Chamber the round, then eject it. If the bolt closes, and the grease is smashed really really flat, odds are your headspace is OK. Clean the grease off chamber and round before resuming use.

Rough rule of thumb: Primer pockets stretch on a single firing at 75,000 PSI. Steel yields a tiny bit at 100,000 PSI. If your primers are staying snug, don't worry about having stretched your action.

I put a layer of masking tape on the back of a case.

One layer is .005".

If it chambers with three layers, I am going through a fireforming process with that brass, or set back the barrel, or something.

In checking and shooting scores of rifles, I only found one problem: A Savage 99 that had just been rebarrelled by "the best gunsmith in the state of Washington" and appeared unfired.
It would chamber with 6 layers.
The seller suggested letting the gunsmith fix it for free.
I declined the help and demanded money.

Red,

Regarding headspace, since both your guns are AI it may be a toss up whether they were chambered to specs or not. Do you have an old fired round somewhere you can use to check them with? The best method I have heard for checking headspace is to disassemble the bolt and remove the firing pin and mainspring, then add cellophane tape to the cartridge base until you feel resistance, then peel the tape off and measure it with a micrometer.

Whether or not hot loads caused a headspace problem depends a lot on what action is used.