Hello from Baltimore

[LRRPF52]

Interesting note about the comparator. I have one, but I have used it sparingly during primarily load development.

From what you are saying, it sounds like I would use a 0.35" bushing to measure at a particular point in the shoulder of the cartridge. I suppose the thought process is that as I use the brass more, the shoulder will move and I can measure that change. But I am not sure what to do with that knowledge in particular. Can you help refine that thought process for me?

Factory ammo and virgin brass comes under-sized so it will fit in the largest number of chambers.

When you fire it, the case will blow out to the maximum dimensions of the chamber (and rebound a little).

The biggest measurable location of case growth is from the base to the shoulder (headspace in bottleneck cartridges). The comparator with .350” bushing will show you how much it has grown, and also will help you as you set up your sizing die for optimum/minimal work-hardening of the brass. (In the SAAMI drawings, the datum line midpoint on the shoulder is a .350” diameter, hence the .350” bushing being used for 6.5 Grendel. The other bushings in the set are for different cartridges with larger or smaller datum line diameters.)

A common mistake reloaders make is to not size the brass enough, or size it too much.

With gas guns, we typically see a .003” shoulder datum bump back from the fired dimension.

You will have to figure this out with your chamber though, using the comparator as you adjust your sizing die.

I have a stripped Grendel AR-15 bolt for this purpose, that has the same bolt face depth and lug lengths as my other bolts.

It’s one thing to see if a re-sized case will drop into a chamber and fall out, but another thing to determine if the bolt will actually close on it and allow the lugs to rotate into lock-up.

It’s harder to determine this with an assembled bolt due to the ejector.