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.
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.