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CBTO and Max COAL - 6.5 Creedmoor

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  • Rathmatik

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    Jun 25, 2019
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    Howdy,

    I recently acquired a Bergara B-14 HMR in 6.5 Creedmoor, and also got the Hornady OAL gauge and comparator setup.

    I just bought some 143gr ELD-X bullets to make some precision and hunting loads. I tested 5 bullets, and came to a consistent 2.243" CBTO. This measures to ~2.890" COAL.

    The HMR has a AICS magazine, which has a max allowed COAL of 2.860", so clearly I need to reduce my CBTO by at least 0.030" off the lands. I figured I might start with 0.040" off the lands and go from there. This would give me 2.203" CBTO and something in the ballpark of 2.850" COAL.

    Now, my question is this. Since Hornady defines the Max COAL for 6.5 Creedmoor to be 2.825", am I required to remain under that length for any reason? Or is that just the SAAMI specification for these short actions?

    Thanks!
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    EZ-E

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    You could go over the SAAMI COAL spec in the short action...as you found out your limited to mag lenth. I have a RAP in 6.5 & an AR10 in 6.5cm both have different maxes for CBTO & COAL. But both are still limited to mag length.
     

    Rathmatik

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    You could go over the SAAMI COAL spec in the short action...as you found out your limited to mag lenth. I have a RAP in 6.5 & an AR10 in 6.5cm both have different maxes for CBTO & COAL. But both are still limited to mag length.

    Okay gotcha. Yep so it seems. Going to run a similar test eventually with my 30-06, which has only an internal mag, and see what I can get away with there.
     

    Younggun

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    There are a lot of variables to what COAL a chamber can handle, from bullets shape to throat length.

    If you know you won’t be cramming the bullet in to the lands you can go over listed max coal. It may or may not improve your groups. You also want to keep a certain amount of contact area of the bullet in the case. Usually about equal to bullet diameter.

    It’s not uncommon for match bullets on the heavier side to fit a chamber while being too long to fit the magazine. I’d recommend experimenting with mag length loads and see if you actually get any improvement by loading long. Some bullets do well with a bit of a jump before hitting the lands.


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    Dawico

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    I agree on starting at maximum magazine length.

    0.020" off the lands has served me well but usually requires single feeding of magazine rifles as that puts the rounds too long.

    Given your throat and magazine measurements 0.030" off the lands is definitely where I'd start and probably stay when it's all said and done.

    You can test further off the lands when you get a powder charge figured out but I wouldn't go longer as single feeding a rifle is a pain.
     

    Rathmatik

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    I agree on starting at maximum magazine length.

    0.020" off the lands has served me well but usually requires single feeding of magazine rifles as that puts the rounds too long.

    Given your throat and magazine measurements 0.030" off the lands is definitely where I'd start and probably stay when it's all said and done.

    You can test further off the lands when you get a powder charge figured out but I wouldn't go longer as single feeding a rifle is a pain.

    Yeah, I definitely don't want to single feed. I bought a magazine fed rifle for a reason haha.

    Thanks for the input. I'll try 0.030" off and see how that works with the magazine. Since that would put the COAL right at the max for the mag, I might knock off another 0.003" to handle bullet length variation. That'd give me a 2.210" CBTO. We shall see how much these bullets vary in length.

    I'll post back once I make the first batch.
     

    Rathmatik

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    Update:

    So I actually had to reduce the CBTO to 2.175” due to the rounds getting caught on the edge of the magazine well. This turned out to be about 2.825” and less OAL, which is interesting.

    Anywho, I tested with 36, 37.5, 39, and 40.5 grains. They all went bang and shot well at this length, with 40.5 being the best group. I’m going to test some more with 40, 40.5, and 41 grains.
     

    Lonesome Dove

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    Some Mags have enough material and are easy to Dremel for longer seating. I’m not familiar with that of the Bergara but it’s easy to determine should you choose to do so.
     

    Rathmatik

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    I use these mags without a binder plate allowing me to load to 2.950. Of course if you're hitting the mag well already that won't do you ant good.

    Ah, that is pretty cool. Yeah I had started at about 2.860" OAL for the stock mag, but the mag well is the bottleneck. It's not a huge deal, still just 0.068" off the lands according to my max CBTO measurement. I suppose if I really wanted to test longer, I could load single rounds into the chamber. But, the accuracy is plenty fine for me so far and I don't like that tradeoff.
     

    dee

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    Be mindful that depending upon action and mag interface that you might still be limited on oal. Some actions are only cut to run 2.850" oal.

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