Measuring to the lands

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by ohiohunter, May 7, 2015.

  1. ohiohunter

    ohiohunter Well-Known Member

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    So I have a sinclair measuring tool to measure my chamber length. From the rear of the action you put a bullet in the chamber and measure then measure the length to the base of a case. Take this measurement and add the bullet length and it should tell you the OAL.

    Now what I did was all the same measurements, but instead of the entire bullet length I measured from the base to the ogive of the bullet. I did get consistent measurements, but it was not repeatable w/ different bullets. Why?

    I am under the impression that using the comparator will give me my base to ogive measurement regardless of the bullet used b/c it is measuring the same point on the ogive as the next. I understand bullet length will effect this, but shouldn't that be factored out when measuring off the ogive vs overall length of the bullet? I expect some variation b/c some bullets sneak into the lands more than others, but I have about a 0.040" range.
     
  2. rbcowboy

    rbcowboy Well-Known Member

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    I dont know exactly how the sinclair works but I have the hornady comparator and if you are measuring different types of bullets you will get different readings because of the different styles of ogives for the different types of bullets.
    You will also get different readings for bullets of the same lot but this variation won't be as drastic as .040
    When reloading for accuracy I measure all my bullets base to ogive and group them accordingly so that when I seat them for my ogive COAL they are also seating the same depth in the case within .001-2
    If if works like the hornady comparator you also have to be sure that you are putting the same amount of pressure pushing it into the lands because you can seat one futher into the lands quite a bit more just pushing slightly harder on the seating rod from the previous bullet!
    Don't take this as law by any means! I'm new to this myself but this is what I have learned along the way thus far! I'm sure one of the veteran guys can give a better explanation and guidance but thought I might give a suggestion to start with!
    Best of luck to you!!!
     
  3. FearNoWind

    FearNoWind Well-Known Member

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    The bearing surface will contact the lands at different points along the taper. The COBL of one bullet will differ from another manufacturer or type depending on overall design. You should be able to obtain fairly consistent measurements when comparing a variety of the same bullets from the same lot but even then you'll find variances. Your home reloading equipment is not laboratory test quality gear. Comparing the bullets of the same caliber from different manufacturers will prove to be an exercise in futility.
     
  4. FEENIX

    FEENIX Well-Known Member

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  5. pods8

    pods8 Well-Known Member

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    I measured up a variety of various 7mm bullets a bit ago for overall length, base to comparator, and comparator to tip (calculated from base to comparator). Mainly I was wondering how a given bullet may sit in the case (roughly) as I pondered out if I wanted to get a rifle throated in a specific manner. Anyways perhaps of use:


    http://www.longrangehunting.com/for...various-7mm-bullets-measurements-bullets1.jpg

    http://www.longrangehunting.com/for...various-7mm-bullets-measurements-bullets2.jpg
     
  6. ohiohunter

    ohiohunter Well-Known Member

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    Part 2 explains my issue. I assumed the comparator would hit all the bullets in the same spot, which it does, BUT each bullet does not engage the lands at the same spot relative to the comparator. Where I get the large discrepancy between a 215gr berger and a 168gr sierra is the pitch of the bullet.

    I should've ordered an extra long barrel then taken the excess and made a comparator out of it, then the ogive to lands junction would be consistent.

    Thank you feenix
     
    Last edited: May 8, 2015
  7. pods8

    pods8 Well-Known Member

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    Sounds like you are still trying to use your findings from one bullet type to dictate how another will hit your lands, that isn't how it works. You figure out where one bullet type hits your lands and record the CBTO that your comparator measures for that length and use that to adjust your seating die to have the best chance at repeatable consistency through that lot of bullet. Call it the 215gr berger.

    If its time to load the 168gr sierras you figure out where those hit the lands and repeat the process, you don't use your measurements off the 215gr berger to try and place the 168gr sierras.
     
  8. FEENIX

    FEENIX Well-Known Member

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    You're welcome!

    Sorry but I think you're still complicating it unnecessarily, each bullet design as noted by others will yield different results, i.e ....

    This measurements (using Hornady LNL OAL gauge/comparator) touching the lands are for my .300 WSM but it should be relevant;

    190 Berger VLD ........................ COAL = 2.928"; CBTO = 2.253"

    208 Hornady A-Max .................. COAL = 2.985"; CBTO = 2.258"

    210 Berger VLD ........................ COAL = 2.917"; CBTO = 2.246"

    215 Berger VLD ........................ COAL = 3.042"; CBTO = 2.277"



    NOTE: Current load (still under development) for 215; COAL = 3.003; CBTO = 2.231 (.046" off the lands)


    [​IMG]

    (L-R: 190 VLD, 208 A-Max, 210 VLD, and 215 VLD)
     
    Last edited: May 8, 2015
  9. ohiohunter

    ohiohunter Well-Known Member

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    I get it, and its not that complicated. I just had to make an explanation of it to refute my previous thinking. I do however believe if your comparator is identical to the diameter of your lands then your measurements will be consistent across all bullets, but obviously it is not. So you're basically measuring off the ogive to determine when the bearing surface contacts the lands (not actually measuring to the bearing surface/lands junction) and due to the different slopes off different bullet designs this number fluctuates and must me determined for each bullet to be used.
     
  10. pods8

    pods8 Well-Known Member

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    Not all barrels in the same caliber have the same dia lands either...
     
  11. ohiohunter

    ohiohunter Well-Known Member

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    Of course, thats why I stated I had wished I'd ordered an extra long barrel to use the excess for a comparator which would be identical to the lands I am measuring for.
     
  12. davkrat

    davkrat Well-Known Member

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    Add a little throat wear and even your theoretical "barrel" comparator would likely be different.

    All you need to do is determine the jump/jam. The measured length for your barrel/bullet/comparator isn't worth a hill of beans other than a baseline measurement for determining your barrel's preferred jump/jam. "A rose is a rose by anyother name."
     
  13. ohiohunter

    ohiohunter Well-Known Member

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    Not likely, it absolutely would... So from your perspective a 0.100" jump is the same as a 0.001" ... I guess a jump is a jump. I'm glad thats all I need to determine. :rolleyes:
     
  14. pods8

    pods8 Well-Known Member

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    I think he is highlighting use the actual bullets to check, use a comparator to try to be consistent and periodically recheck as throats wear and certainly recheck with different bullets. A comparator is just a baseline measurement for one set of circumstances.