Case Head to Datum Line Measurement, 6.5 Creedmoor

Checked 34 cases of Lapua 6.5 Creedmoor brass this morning using a Hornady Lock-N-Load Headspace gauge to check the Case Head to Shoulder Datum Line Measurement. New Lapua Brass consistently measures 1.555". These particular cases were reloaded 3 times and the shoulder bump that I used last time produced a measurement of 1.555". I measured the fired brass this morning and came up with the following: 1.5575" (5 cases), 1.558" (18 cases), 1.559" (8 cases), and 1.560" (3 cases). I would think the formed-fired cases would be closer in dimension. I have always annealed this brass, starting with the first loading. Is this variation, .0025" within normal expectations? I'm going to strip my bolt even though it is a bit of a PITA, and check my chamber dimensions with this fired brass and see what the actual headspace of the chamber measurement is.

The fired cases are not fully formed to your chamber. Not a big deal, just back your die out so your longest case is reduced about .003 ( 1.560-.003 = 1.557) for example and monitor from there. If you don’t get bolt lift resistance you’re good to go, if you do then adjust your die in a .001

Thanks for the guidance, I'll give it a try.

I notice that I get varying datum lengths depending on how I apply the lube. Go figure.

^With the Hornady Headspace gauge it is hard to get exact measurements.... it does good enough to get you in the correct range- but if you measure cases multiple times (without looking at the display) you will get different results. When you look at the display you can game it to get the result you want.

Even when the cases are fully formed to your chamber some will read slightly different. They expand when you fire and then contract as they very quickly cool.... and they don't always contract the exact same amount.

Or it could be the $29 calipers you are using....lol

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Originally Posted by Whynot

^With the Hornady Headspace gauge it is hard to get exact measurements.... it does good enough to get you in the correct range- but if you measure cases multiple times (without looking at the display) you will get different results. When you look at the display you can game it to get the result you want.

Even when the cases are fully formed to your chamber some will read slightly different. They expand when you fire and then contract as they very quickly cool.... and they don't always contract the exact same amount.

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Very true. Also, we are mostly kidding ourselves trying to measure to a half of a thou. I least I am. Haha.

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Originally Posted by sharpshooter

Or it could be the $29 calipers you are using....lol

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It could be- but isn't. The first set of calipers that I started with were probably cheaper than that... and then "upgraded" to some hornady branded ones that probably cost around $30- but still about the same quality because putting Hornady on the side had to jack the price up a little. But for work I purchased some Mitutoyo ones that would have been around $150 and although they are much higher quality you still get different readings just because of the materials being used- how they interface and small variances in pressure. Not that it matters....

Most of my measurements are close enough. Rgr on seeing variations. It's all part of the game. Whatta Hobby!

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Originally Posted by Whynot

It could be- but isn't. The first set of calipers that I started with were probably cheaper than that... and then "upgraded" to some hornady branded ones that probably cost around $30- but still about the same quality because putting Hornady on the side had to jack the price up a little. But for work I purchased some Mitutoyo ones that would have been around $150 and although they are much higher quality you still get different readings just because of the materials being used- how they interface and small variances in pressure. Not that it matters....

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I used an RCBS precision mic for a long time. After using the Hornady Aluminum stuff for a while, I went back to the RCBS precision mic.

Attachment 10326

How do they shoot? Are the shorter/longer ones giving you fliers?

When I find a variance (measuring case, change in bullet tension, velocity excursion, POI excursion, etc) I mark the case and then track vel and POI to see if it is different than the rest. If not I keep shooting it. If it is 'off' a second time I toss the case.