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What is my length to lands?

 
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Old 05-05-2013, 07:56 PM
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Join Date: Dec 2007
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What is my length to lands?

Working up loads for my new 300 win mag. (Will post pictures soon) Using a hornady overall length gauge and comparator with a hornady modified 300 win mag casing I am measuring the length to lands with a 230 berger hybrid bullet. I am consistently coming up with 2.902" from base to ogive. But my question is, after I fire a case in my chamber it reads 2.270" from base to datum of shoulder and I bump that back to 2.269". On this hornady modified case from base to shoulder measures 2.258" meaning it is .011" shorter than what I will be actually firing. I know the best way to solve this problem would be to modify a once fired casing but until I do this what do I seat my bullets to for touching the lands? I have been crunching numbers and running scenarios on this for a while now and still cant figure it out. Also because I am headspacing my brass off the shoulder not the belt, is this why it is so far off? And even if so, how do you account for this? What I'm trying to figure out is do I add .011" to my 2.902" meaning on a properly sized case touching the lands would be 2.913" or is this not right because something to do with the belt coming into contact before the .011" longer shoulder would?
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Old 05-05-2013, 08:25 PM
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Join Date: Nov 2010
Location: deer park, wa 99006
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Re: What is my length to lands?

Add the .011
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Old 05-05-2013, 08:27 PM
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Join Date: Dec 2010
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Re: What is my length to lands?

I use a 1/4 dowel that is a little longer than the barrel to face of the bolt.

Stick the dowel in the barrel with the bolt in and record the distance the dowel protrudes.

Shove the selected bullet in the barrel and hold it against the lands and measure the dowel protrusion.

Subtract the first from the second measurement and that is your OAL to the lands. I just zero my caliper on the first and then read the oal directly as the second measurement.

Then you can place that same bullet in a case to the depth you want and measure with a comparator to get the COAL and actual distance off the lands by repeating the process with the same bullet.
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Old 05-06-2013, 11:13 AM
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Join Date: Dec 2005
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Re: What is my length to lands?

The only good place on a case to measure the distance the bullet jumps to the lands is its shoulder. That's hard against the chamber shoulder which is a fixed distance back from the lands; when the barrel's new anyway. Then it lengthens as the leade wears/erodes away.

Measuring from the case head introduces all sorts of variables.

A few thousandths spread is normal anyway. No batch of bullets have exactly the same ogive shape. They all vary a tiny bit 'cause the jackets and cores are not dimensionall perfect.

Use an approximation within a few thousandths and leave it that way. You'll never know the difference unless you can shoot your stuff into no worse than 1/10th MOA at 1000 yards anyway.
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