Question regarding measuring headspace

[BravoSix]

I've been reloading for handguns for years, but I just recently started handloading for precision rifles. I bought the Hornady headspace guage and I'm getting some varying data. I am reloading for a rem700 with the factory .243win barrel. I am using digital calipers that I've had for over a decade and that I am confident are reliable. When I measure the deprimed fired cases using the hornady guage, I am getting measurements between 1.6215" and 1.6255". I've tripple-checked the measurements and they are correct as far as I can tell. Most people suggest bumping the shoulder back 1 to 3 thou. My prior loads I've set my sizer to bump the shoulder back to 1.620", based on the shorter of the fired cases. However, I am questioning whether I might be over-sizing my brass. Can someone give me some input on if I should size based on the shorter numbers, the longer numbers, or take an average?

FWIW, I tried chambering the fired cases and did not notice a great deal of difference in effort it took to chamber the longer cases versus the shorter ones.

 

[Beluebow]

Is this once fired brass?I would bump the 1.6255 back .0015. the rest of the brass will catch up eventually and you wouldnt be overworking it.

 

[rsplante]

I'm struggling through an OAL issue on a parallel thread. You mention basing a measurement on the shorter of your fireformed cases. If you think about it, the longer case has to be the more correct one because a fireformed case cannot possibly be longer than the chamber of your rifle. With one possible exception that I can think of. I measured my fireformed cases with the RCBS Precision Mic Cartridge Headspace Tool and while writing my post, I measured seven cases and one of them was 0.003" longer than the rest of them. How could that be? When I subjected the errant case to further inspection, I found that the extractor had kicked up a divot on the head, and when I filed it flat, voila, the case measured the same length as the others.

 

[BravoSix]

Is this once fired brass?I would bump the 1.6255 back .0015. the rest of the brass will catch up eventually and you wouldnt be overworking it.

Thanks for the feedback. Yes, the batch I was measuring tonight were once-fired RP cases from handloads. I speculate that the variance might be due to variable charge weights during load development. However, I also noticed some variance from factory loads.

I will adjust my sizing die to size to 1.624.

 

[BravoSix]

I'm struggling through an OAL issue on a parallel thread. You mention basing a measurement on the shorter of your fireformed cases. If you think about it, the longer case has to be the more correct one because a fireformed case cannot possibly be longer than the chamber of your rifle. With one possible exception that I can think of. I measured my fireformed cases with the RCBS Precision Mic Cartridge Headspace Tool and while writing my post, I measured seven cases and one of them was 0.003" longer than the rest of them. How could that be? When I subjected the errant case to further inspection, I found that the extractor had kicked up a divot on the head, and when I filed it flat, voila, the case measured the same length as the others.

I spin my cases in the caliper to try to account for something like this, but I will double-check to make sure this isn't skewing my numbers.

 

[FLIGHT762]

I've found that brass measurements will vary at times due to the work hardening of the brass and the type and amount of lube used in resizing. You could try annealing and see if that helps in getting more consistant readings in your fired brass.

In resizing, I've also noted if you run the case quickly through the F/L die, you can get different readings with less shoulder bump than running the case slowly and leaving it in the die for a second or two before removing it. One other thing I have done is to run the case fully into the F/L die, retract the case about an inch (before the sizing button goes through the neck) and run the case again fully into the die a second time. This will sometimes give more consistant shoulder bumping. I did this a few days ago on some L C 5.56 and it worked well, giving consistant shoulder bump measurements.

See if any of this helps. In the meantime, that .004" variable isn't going to wear you brass out.

 

[rsplante]

I spin my cases in the caliper to try to account for something like this, but I will double-check to make sure this isn't skewing my numbers.

I understand what you are saying; however, in this particular case it would have no effect since the RCBS Precision Mic Cartridge Headspace Tool measures to the entire head unlike a caliper. RCBS Precision Mic Cartridge Headspace Tool | Brownells