Join TexasGunTalk

CBTO and Max COAL - 6.5 Creedmoor

Discussion in 'Reloading' started by Rathmatik, Feb 12, 2020.

  1. Rathmatik

    Rathmatik Member

    72
    18
    8
    Jun 25, 2019
    Dallas
    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!
     


  2. EZ-E

    EZ-E Freedom vs Tyranny 2020 TGT Supporter

    3,395
    5,905
    113
    May 4, 2017
    Cypress
    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.
     
  3. Rathmatik

    Rathmatik Member

    72
    18
    8
    Jun 25, 2019
    Dallas
    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.
     
  4. Younggun

    Younggun Ginger Avenger TGT Supporter Admin

    50,137
    8,105
    113
    Jul 31, 2011
    hill co.
    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.


    Sent from my iPhone using Tapatalk
     
    Rathmatik and EZ-E like this.
  5. Dawico

    Dawico Uncoiled Lifetime Member

    33,788
    12,059
    113
    Oct 15, 2009
    Lampasas, Texas
    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.
     
  6. Rathmatik

    Rathmatik Member

    72
    18
    8
    Jun 25, 2019
    Dallas
    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.
     
    EZ-E and Dawico like this.
  7. Rathmatik

    Rathmatik Member

    72
    18
    8
    Jun 25, 2019
    Dallas
    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.
     
  8. Lonesome Dove

    Lonesome Dove Active Member TGT Supporter

    665
    581
    93
    Sep 25, 2018
    Texas
    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.
     
  9. orbitup

    orbitup Sticker Cop TGT Supporter

    21,147
    15,097
    113
    Nov 6, 2010
    Waxyscratchy
    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.
     
  10. Rathmatik

    Rathmatik Member

    72
    18
    8
    Jun 25, 2019
    Dallas
    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.
     


Share This Page