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Thread: Measuring OAL

  1. #1
    Member martentrapper's Avatar
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    Default Measuring OAL

    I've been reloading for my 6.8 ARs. Max OAL for them is 2.3 in. because of the magazine size. I have been measuring OAL of the finished cartridges with a regular dial caliper. I notice differences with plastic tipped bullets.
    The other reason OAL is important is to predict bullet jump. I understand that minimal bullet jump gives better accuracy, but have also read you can vary OAL to obtain the best accuracy for the round. Apparently, I should get an attachment for the caliper that seats onto the bullet to measure OAL.
    What do you folks recommend for this?
    Thanks!
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  2. #2
    Member marshall's Avatar
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    Quote Originally Posted by martentrapper View Post
    Apparently, I should get an attachment for the caliper that seats onto the bullet to measure OAL.
    What do you folks recommend for this?
    Thanks!
    There are a few different systems, I've had good luck with the Hornady version. It's quite simple to use and repeatable. There is a fine fella on here that hates it but I'm sure I could change his mind if I had a few minutes to help him get it figured out...

    It measures from the head to the ogive. The insert is a few thousands smaller than the bore. Simple use a fired case attached to a tool and run a bullet up to the lands. Lock it down and measure as shown. This is the equivalent zero lands OAL. I say equivalent because you are actually measuring to a point that you can repeat based on the distance the sample was taken.

    If you use a purchased modified case simply measure the difference between the head and shoulder of it versus your fired brass. Add that distance to the ogive measurement and your back at zero.

    http://www.hornady.com/store/Lock-N-...traight-1Each/

    http://www.hornady.com/store/Modified-Cases/

    The head space set works in a similar fashion. It measures from the head to a mid point on the shoulder. You can run your die down in small steps until it just bumps the shoulder and take a real measurement with this system. I like a .002 shoulder bump/set back rather than full length resizing. It provides for a reliable feed and smooth bolt closure with minimum brass work. It extends the life of your brass and improves accuracy.

    http://www.hornady.com/store/Headspace-Gauge-Kits/

    If you're reloading for semi auto rifles it's probably best to F/L re-size. You can still adjust seating depth for improved accuracy.

    Click image for larger version. 

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  3. #3
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    Wow. What marshall said. I use the Hornady system also, because two different vendors in Fairbanks carry parts for it.

    Also, if you find a good factory load you may want to measure those up since your rifle seems to like that head to ogive length...

  4. #4

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    Quote Originally Posted by marshall;1206997

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    65642[/ATTACH]
    +1, also once you know how far an ogive sets off the lands changing bullets and loads becomes easier. As stated though your magazine may become your limiting factor. you should however have no bullet jump if you taper crimp your ammo.
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  5. #5
    Moderator LuJon's Avatar
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    Default Measuring OAL

    I don't mess with my 6.8 AR since the mag length is so much shorter than my chamber that there is nothing that will fit in the mag that would be to long. To me the ogive measurement on it is a waste since the tips vary between rounds and it is the tip that must clear the front of the mag.

    I also use the hornady COAL tool and even went so far as to order the correct die to mod my own cases.

  6. #6
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    What are you trying to measure?

    You're rifle, ?? many ways to do that.

    Loaded round, ?? I suggest, Sinclair Nut. (It looks like one, with bore size holes on the flats.)

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  7. #7
    Member martentrapper's Avatar
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    I want to check a couple 6.8 ARs. I know the mag is the limiting factor but can still vary coal to possibly effect accuracy. Just ordered some stuff from Midway so I'll see how it works out.
    I can't help being a lazy, dumb, weekend warrior.......I have a JOB!
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