Headspace on JP LR07 308 Win

[verdugo60]

So I am not new to reloading but am new to loading for a precision semi-auto. I am helping a buddy work up some loads for his JP, with once fired LC Fed brass. I am using my new Hornady Headspace gauge with the proper insert and know for a semi-auto I should be bumping back .003-.005".

With my bolt guns it's easy to just bump the shoulder until the bolt barely closes, but with JP I don't have that luxury of "feeling" the bolt.

My issue is that the brass itself is not giving me a PRE-sizing headspace measurement that is consistent.

First three fired cases I measured were:

1.626"
1.632"
1.628"

So when setting my Redding Die (non-bushing, FL die) I can set it to bump the first case back to 1.623" but that won't necessarily bump the next case that is 1.632" back .003-.005 right?

Perhaps I'm overthinking this and need to just find an average that chambers well (again, harder to tell when I have not trimmed yet and some cases are running over spec on OAL after sizing (2.2") I just want to avoid over-working the brass and causing possible case-head separation down the road. I also don't want to not size enough to where some won't run consistently in the gas gun.

Set me straight please!

 

[Brux]

If you are measuring these cases with the fired primer still in them you might get different measurements like you are getting and if you set your FL sizing die up to size at 1623 it should size all the cases to that.

 

[FLIGHT762]

As Brux suggested, when you measure your fired cases for headspace bump, you should deprime them all first. Sometimes, the primers will crater and will give you false, higher readings. When shooting cases from an autoloader, be sure to inspect the case heads and run your finger across the base and look to see it there are any scoring or burrs caused by the ejector. This can also give you false readings.

 

[427Cobra]

Yep deprime first, IMHO your samples are not enough to correctly determine FL die setup, and I would recommend that the longest measured case/cases be used to determine FL die setup.

 

[bigedp51]

Also on over gassed rifles the bolt can start moving to the rear while pressure is still in the barrel, this can cause the case to stretch and end up "longer" in headspace than the chamber is. This is on top of the uncrimped primer backing out, remove the fired primers with a universal decapping die and remeasure the cases.

You can check your chambers headspace with a new or completely full length resized case and a fired spent primer.

1. Measure the case total length and write it down.

2. Now just using your fingers start the primer into the primer pocket.

3. Now slowly chamber this test cartridge letting the bolt seat the primer as the bolt closes.

4. Now remove the test cartridge and remeasure its total length again and write it down.

5. Now subtract the first case measurement from the second case measurement and this is your head clearance or the "air space" between the bolt face and the rear of the case. You can use this measurement for adjusting for the correct shoulder bump or head clearance.

You can also use a Hornady cartridge case headspace gauge measuring from the base of the primer to the shoulder to get your actual chamber headspace length to adjust your dies.

You now know how to make the cheap bastards headspace gauge.

 

[verdugo60]

Thanks to everyone for the suggestions, you guys rock. Really appreciate it. We took the JP to the range and put 100 flawless reloads through! Gun shot better than it ever has. I think with a match, two stage trigger added it would be around a .75 MOA shooter on average and get close to .5 MOA sometimes. I had a .4 MOA 4 shot group until I blew it with a 5th flyer! Gas guns are different beasts for sure.