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The once fired brass gives an impression of my rifles headspace. Both once fired 2.8195, and several others measured within 1/2 thou of this.
My Lapua FL resized from the batch that I had a lot of trouble with 2.8125, fully 0.007 shorter at the shoulder. As Obsessed and Chuck have said, this was my issue.
I had no idea that I’d bumped the shoulder this far, nor did I even know what number would constitute pushing it too far. I was just following RCBS instructions. That was lesson 1 for me.
Lesson 2 was more nuanced: I mentioned about my confusion at why FL resized brass with the die set at any point less than 1/8 turn past the point of contact did not allow the resized brass to chamber without significant pressure on the bolt as it was depressed. I asked Obsessed about this, since I couldn’t figure it out.
It turns out that as the ram is brought back up and the expander ball runs back through the neck, when the neck is not properly lubricated the ball will create a bump here, which effectively bumps the neck/shoulder junction forward.
Even though I run a lubed nylon brush through the neck, I believe that this is what was causing the otherwise-properly bumped shoulders to fail to chamber.
So with that in mind, my next step is to try a better graphite style neck lube, and (as pretty much everyone here has suggested) aim to bump 0.002 on resizing by checking with the 9mm brass method and going for a value of 2.8195 (rifle headspace) - 0.002 = 2.8175.
Makes sense?
Thanks fellas. I now know FAR MORE about this stuff than this time last month.
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