Headspace is simple. It’s the distance from the gun’s bolt face to the surface inside the chamber that stops the forward motion of the cartridge as it’s inserted. In a bottleneck cartridge, the case headspaces on the shoulder.
Many shooters, and all reloaders, know that. But I think there’s a misunderstanding of case length (maybe it’s just my misunderstanding). We’re told in all the manuals to carefully check the length of our cases before reloading, and to trim them if they’re beyond a certain specified length.
Actually there are two important case lengths to a bottlenecked case. The distance from the head to the shoulder, and the distance from the shoulder to the case mouth.
My Winchester has what I regard to be excessive headspace, which means that if I fire a factory load, the case will stretch backward, to fill the extra space. I suspect most of the stretching is between the shoulder and the bolt face. If I neck size the case, or size it so the shoulder is pushed back only a thousandth or two, the case is now “too long” and I am told, in all the loading manuals, to trim it. That would be shortening the neck in response to stretching behind the shoulder, and it would accomplish nothing.
Sure; the cases being loaded should all be the same length so they’re crimped equally, but I won’t know how far the case’s neck extends into the chamber by just measuring the overall length of the case. Once I have a case that’s fire-formed to my rifle’s chamber, the neck may or may not need trimming.
Are there case gauges that make it easy to take a measurement of the mouth-to-shoulder length as well as the headspace? Should I just shut up and full-length size all my cases, trim them to spec, and wait for the cases to deteriorate from excessive stretching and sizing? (the rifle was checked by a “gunsmith” and declared to be within spec, BTW)