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Headspace Datum Line?

 
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  #1  
Old 09-29-2012, 03:15 PM
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Headspace Datum Line?

What is the headspace datum line measurement for the 338 Lapua? I hear the .420 "E" bushing for the Hornady tool is not correct? Should be .45....something?
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Old 10-01-2012, 04:36 AM
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Re: Headspace Datum Line?

Is it .458???? Anyone know for sure? Need the correct insert for my Hornady/Stoney headspace comparator tool.
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Old 10-01-2012, 10:07 AM
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Re: Headspace Datum Line?

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Originally Posted by Ludicrous View Post
What is the headspace datum line measurement for the 338 Lapua? I hear the .420 "E" bushing for the Hornady tool is not correct? Should be .45....something?
a gauge line can be whatever the guy drawing it up sets it at. Splitting the shoulder O.D. is the way I always figure it, but you could do it with a .421" number or even a .470" number. Then you also have to take into fact that chambers will vary a few thousandths here and there. I always made my own bushings to fit individual cases and chambers, and my calculations say .4575" is the number for the bushing I.D. But a .421" bushing should work just fine.
gary
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Old 10-01-2012, 08:59 PM
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Re: Headspace Datum Line?

"What is the headspace datum line measurement for the 338 Lapua?"

It really doesn't matter so far as the Hornady gauge goes, it'a just a relative measurement point anyway. Anything that stops somewhere on the straight part of the shoulder will work fine, meaning the common .338 case shoulder bushing will work quite nicely no matter what the SAAMI spec datum is.
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Old 10-02-2012, 03:19 AM
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Re: Headspace Datum Line?

Roger that. I'm bumping shoulders back .002 and wasn't quite sure if it made any difference at all where the measurement is taken. Thanks.
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Old 10-02-2012, 10:37 AM
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Re: Headspace Datum Line?

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Originally Posted by Ludicrous View Post
Roger that. I'm bumping shoulders back .002 and wasn't quite sure if it made any difference at all where the measurement is taken. Thanks.
I always CAD out a once fired case (actually five cases), and setup my own set of numbers. Try to split the difference between the neck O.D. and the shoulder diameter; give or take .010". This becomes my own gauge dine and I then calculate the distance from the face of the case to the gauge line. Thus creating the gauge line deminsion. I may adjust the length a couple thousandths as things begine to settle in, but rarely more than four thousandths, and usually less than two. I donot trust generic headspace gauges for this as chambers will vary slightly. You culd probably use something like a 12mm or even a 7/16" inner race from a needle bearing as long as it's longer than the neck. (you know right upfront that the faces on an inner race are parallel). Or simply make your own in a lathe.

You are correct in bumping the shoulder back .002" to .003", but that number becomes generic if you base everything off an existing drawing. I prefer to do this by chambering a case in the rifle looking for a slight bit of resistence while I close the bolt. Then I bump everything back from there. I also will vary the amount of bump per individual chambering. (I only bump .222's and .223's back about .001" max). I keep a log book on what I do here as I'm getting old now!

I might also add here (although slightly off topic) that one of the first things I do is to find out just how long the actual neck is in the chamber, and setup a max overall length from that distance; minus .015" (I hate trimming cases almost as much as I hate reaming primer pockets)
gary
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Old 10-02-2012, 12:29 PM
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Re: Headspace Datum Line?

Thanks Gary. I also base my calculations off fired brass from that specific chamber, if I have 50 fired cases I measure ALL of them and line them up in files, then whichever file has the most cases I base my shoulder setback on those. I also dislike trimming and measure my chamber lengths with the Sinclair gadget in a shortened case.

I'll measure up my fired cases with your OD neck vs. shoulder average and see where that puts the datum line for may cases.
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