Headspacing with spent primers ...

[Blue68f100]

The point I was making is that a single firing is not going to be a accurate representation of the chamber. You need to neck size and fire it at least 3 times to get a good fit to the chamber. You can force the shoulder back with a bolt gun more so than a simi-auto. Like you said you need to remove the ejector and extractor to get a good reading though. The thing about brass is that it will spring back if you did not go far enough to yield the brass.

 

[rskent]

More commonly, I simply slide shim stock against the base of the case, or even use tape as shimstock, instead of using the spent primer as a depth gauge. The end result is the same no matter what method you use.

Or metal duct tape from the Home Depot.