Hey woolleyworm!
I've got a number of Mosins that I would like to perform a headspace check on, and your's sounds like a quick, great way to do it.
Just a couple of questions:
1. When you say to place the tape on the "headstamo", could you please clarify what part exactly you're referring to? Is this another term for the boltface?
The sticky side of the tape will be placed on the fired cartridge's headstamp, ya don't want to get your boltface gummed up.
2. When checking, should a round be chambered as well, or is this just to be done with an unloaded rifle?
You will be using a fired cartridge and chambering it with the two different thicknesses of tape. You don't need to pull the trigger, just chamber it and close the bolt, you can run the round from the magazine if you prefer.
I find it good practice to have all live ammo sealed up and put away when I'm doing any cleaning/action testing/trigger adjustments/etc.
Thank you very much!!