Different headspace readings.

[Daymar]

Hello everyone,

I’m new to reloading and had a question about headspace. I have some once fired federal cases that I was about to run through a FL resizing die. I measured the headspace using a Hornady headspace comparator and noticed I there’s .004 difference in reading from the biggest to smallest headspace reading. All the cases measured were de primed before taking the measurements using a universal de priming die. All the cases were fired from the same rifle (semi auto AR-15) And are the same brand. Is there a reason for the inconsistent readings and if so which measurement do I use to set the shoulder back?

I’m setting the should back .004.

 

[TheOfficeT-Rex]

Different pressure on the calipers can cause reading inconsistencies. Also try measuring the case, then turning it 90* and measure again. And sometimes, shoulders don't blow all the way forward on the first firing.

I'd use the longest case reading, bump it back 2 or 2 a half thou, then plunk test the brass in your barrel.

I find that with my AR-15 (223 wylde chamber), I'm fine FL sizing with a 2 thou bump.

 

[Daymar]

I try to apply the same pressure and measured the same case several times to make sure it wasn’t that. I also rotate the cases and ended with the same results.

I reloaded 20 rounds previous to noticing this. When I measured the headspace for those rounds I only measured one case thinking headspace would be the same for the rest of the cases and bumped it back 4 thousands. Turns out the measurement I took of the round in the previous batch matches the longest measurement I’m getting now. All of 20 rounds chambered and fired without a problem. I’m thinking of doing as you said and sticking to the longest one which turned out to be the same as the measurement I got on the previous batch I reloaded.

Also, I dropped it in a wilson case gauge and it was below max and above min.

 

[Daymar]

No ejector swipes that I saw and like I said I rotate the case a couple times to make sure the reading stays consistent.

 

[Dustysa4]

Spin and/or slightly wiggle the brass in the gauge until you've obtained the smallest reading. Also, pinch directly on the blades to close the calipers tighter. On my cheap calipers I get some flex (variance) if I use too much force on the wheel.

 

[TheOfficeT-Rex]

Also, I dropped it in a wilson case gauge and it was below max and above min.

Fired case, or sized?

I think you're on the right track now, using the longer measurement. I wouldn't bump them 4 thou myself, but it won't cause any issues chambering or firing if you do.

 

[Howland]

There is almost certainly variation in case headspace on new brass and not every case will expand the same, especially if they aren't fully forced against the chamber shoulder.

Back out your die 1/8 - 1/4 turn (9-18 thousanths) so you won't set the shoulder back any and run your longest headspace case through. Then measure again because the head to shoulder length will increase.

Then remove the upper, hook the rim under the extractor, line the case up and see if you can slide the BCG to full bolt lockup with just light finger pressure. If you can, your cases haven't fully expanded yet. This may take 2-4 firings.

If you can't get to bolt lockup, then incrementally screw the die down. A 5° turn on a 14 tpi die is almost exactly one thousanth of an inch. I use a sharpie to mark the lock ring and die body and a compass to measure the turn. When you get to bolt lockup with just light finger pressure, tighten the lock ring and the die is perfectly adjusted to your chamber with that particular die, press and shellholder.

 

[Daymar]

Fired case, or sized?

I think you're on the right track now, using the longer measurement. I wouldn't bump them 4 thou myself, but it won't cause any issues chambering or firing if you do.

I dropped the sized case in the case gauge.