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Hex Nut Comparator for Measuring Headspace

I got a wild hair the other night while loading some 6mmARC loads. I have the Sinclair hex nut comparator, and use that for seating bullets a specific distance from the lands in my rifle. When used with my calipers, it gives a number that I can reproduce with other bullet profiles to get a specific distance to the lands, and we all know that. I was loading Sierra 85 BTHPs, and Speer 75 HP bullets, and the setting on the seating die is very different to give the same "base-to-ogive" reading using the comparator. Of course, that is it's job.

But out of curiosity, I put a 6ARC case into the hole for 30 caliber, neck first, and it settled about halfway down between the neck and the body, almost centered on the shoulder. I measured a few pieces of fired brass and got almost identical readings, then measured resized brass and it gave me exactly 0.002" less. Since that two-thousandths bump seems to be the magic for bolt guns, I felt pretty good about it. The caliper gets the case nice and perpendicular to the flat on the comparator when you measure, so it makes sense that it was so reproducible. Could be just a crazy coincidence though.

Now I have no idea where on the shoulder a headspace measurement is taken from, meaning exactly where that datum line is, but my 6mm cartridge cases worked well in that 30 cal hole, and I'd like to have this idea picked apart a little. Can I use this hex nut thing to measure headspace? Tell me why this isn't a good way to go.
 
Absolutely can be done that way. You are finding a relative number for your chamber and that's all any of them do, If you don't want to be fancy at all you can use a 7.62X39. Insert the ARC neck and you can get your reference number. for bigger stuff you can find a cartridge that works fine. As long as it hits the shoulder you are GTG. You don't need to hit the datum line. It's all relative to where you start. If the shoulder bumps back 0.002 in ne place it will be doing the same on the rest of the shoulder. Be sure and use the SAME fired case when you are working as various cases will be opened up and not all will be the same from chamber to chamber and load. I just kept mine in an empty DILLON storage box. I now use the Sinclair system having moved on from the Hornady set that works well but I feel is inferior to my current rig.
 
Thanks for that drawing @Doom.
It shows a diameter of .350" for that datum line, and my 30 cal hole is much smaller than that, so I'm measuring closer to the neck than the shoulder. I'm using Redding neck bushing dies, so no worries about the expander pulling on the neck a bit.
I'm going to get a set of the headspace gauges, just seems like that $50 is going to be money well spent, but it felt nice when I made my measurements and learned that my dies were set up to do the .002" bump.
Maybe I'll buy a lottery ticket today too!
 
I got a wild hair the other night while loading some 6mmARC loads. I have the Sinclair hex nut comparator, and use that for seating bullets a specific distance from the lands in my rifle. When used with my calipers, it gives a number that I can reproduce with other bullet profiles to get a specific distance to the lands, and we all know that. I was loading Sierra 85 BTHPs, and Speer 75 HP bullets, and the setting on the seating die is very different to give the same "base-to-ogive" reading using the comparator. Of course, that is it's job.

But out of curiosity, I put a 6ARC case into the hole for 30 caliber, neck first, and it settled about halfway down between the neck and the body, almost centered on the shoulder. I measured a few pieces of fired brass and got almost identical readings, then measured resized brass and it gave me exactly 0.002" less. Since that two-thousandths bump seems to be the magic for bolt guns, I felt pretty good about it. The caliper gets the case nice and perpendicular to the flat on the comparator when you measure, so it makes sense that it was so reproducible. Could be just a crazy coincidence though.

Now I have no idea where on the shoulder a headspace measurement is taken from, meaning exactly where that datum line is, but my 6mm cartridge cases worked well in that 30 cal hole, and I'd like to have this idea picked apart a little. Can I use this hex nut thing to measure headspace? Tell me why this isn't a good way to go.
I know how to accurately determine touch, however I don't consider it a magic number. It's just a starting point to try different seating depths. My main concern is I don't want a bullet stuck in the bore if I extract a bullet.
 
Here's something to consider. I see this occasionally. I just finished a 35 Whelen. Headspace is correct but there is an interference fit on the shoulder. See pic. Caused by a mismatch of the radius at the neck/shoulder junction. That will go away when fired but measuring close to the neck brings this problem into play. Comparing sized and unsized may not be an apples to apples comparison. YMMV

Dave

20241004_091650.jpg
 
But out of curiosity, I put a 6ARC case into the hole for 30 caliber, neck first, and it settled about halfway down between the neck and the body, almost centered on the shoulder.

I do that as well, for straight 6BR. Works well enough for a comparison (makes sense - that's what a comparator is supposed to do.)
 
I believe Hornady tells you that for a case that they don't specifically list, add your neck and case body diameters (just before the shoulder starts on tapered cases) and divide by 2. Use the insert with the size closest to your result.
The idea is to be half way between body and neck diameters, which should be half way down the shoulder.
 
Thanks for that drawing @Doom.
It shows a diameter of .350" for that datum line, and my 30 cal hole is much smaller than that, so I'm measuring closer to the neck than the shoulder. I'm using Redding neck bushing dies, so no worries about the expander pulling on the neck a bit.
I'm going to get a set of the headspace gauges, just seems like that $50 is going to be money well spent, but it felt nice when I made my measurements and learned that my dies were set up to do the .002" bump.
Maybe I'll buy a lottery ticket today too!
All of the headspace comparators are just that. They are not accurate enough to measure the exact dimension of the datum but can be used to compare between measurements made with it. It is not necessary to be accurate and any measurement on the shoulder will work. Hornady actually recommends some cartridges use a different bushing than the one that corresponds to the actual datum dimension.
 

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