6.5 CM CBTO measurement

Discussion in 'Reloading Forum (All Calibers)' started by gjb89, Mar 21, 2019.

  1. gjb89

    gjb89

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    Fairly new to reloading and I am measuring the cartridge base to the ogive/lands junction with the hornady OAL gauge and mitutoyo calipers. I'm getting a spread of readings from 2.2185" to 2.2495" with the 140 GR Hornady ELD-M. Rifle is a Defiance action and BT barrel. Any of these numbers look ok? Also how far off the lands are you loading the 140gr ELDs?
     
  2. Ned Ludd

    Ned Ludd Silver $$ Contributor

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    You should be able to get reproducible numbers with the Hornady OAL gauge. Typically for 10 measurements with the same bullet, I would expect no more than average +/- .002" with a little practice, probably even better. Your are currently talking about a total variance of .031" from high to low, which is totally unacceptable for use in seating depth determination. My suggestion would be to practice using the tool with a single bullet, learning to feel the light "touch" when it first encounters the lands, until such time as you can routinely and reproducibly obtain the same measurement +/- .001" or .002".
     
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  3. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew

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    That doesnt sound right for a comp match. What size insert are you using?
     
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  4. RegionRat

    RegionRat

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    Agree with the others... that your reading variation is way too large.

    To be clear, are you measuring your CBTO on a cartridge, or taking readings on the rifle’s chamber?

    Many times the details of the chamber reamer can make getting a steady reading on the rifle a challenge, but if we are talking cartridges something went wrong.
     
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  5. BeefyT11

    BeefyT11 Silver $$ Contributor

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    Hornady told me that they have heard a lot of great feedback with that bullet shooting .015 to .030 off the lands
     
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  6. SPJ

    SPJ Silver $$ Contributor

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    What’s the difference between a 6.5 Creedmoor and a 6.5 comp match?
     
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  7. Bc'z

    Bc'z Gold $$ Contributor

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    Marketing!!!!!
    I looked it up once, it just never took off also comes in 6mm.
     
  8. SPJ

    SPJ Silver $$ Contributor

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    No difference in chamber?
     
  9. Bc'z

    Bc'z Gold $$ Contributor

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    Yes, I tried to bump a thread for ya, but not seeing it.
    Search 6mm competition match.
     
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  10. SPJ

    SPJ Silver $$ Contributor

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    I think I got it
    The Creedmoor is based on a 30 TC
    The CM is from a 243 parent case
    J
     
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  11. gjb89

    gjb89

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    5-26 insert.

    I tried using a little less pressure and I was able to bring the reading alot closer.
     
  12. McHaggis

    McHaggis

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    With the 140 ELD-M’s measured with the 5-26 insert my Howa 6.5CM is 2.271” just touching. I’m still playing with seating depth but 0.015” and 0.003” both look good.
     
  13. Fliers

    Fliers Gold $$ Contributor

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    I wouldn't expect such a large change even with pressure.
    Is this a single bullet giving you this variance, or is this measuring multiple loaded rounds?
    Is your neck tension too light? Maybe you are pressing the bullet into the case.
    You need to spin the bullet a little to get it flat and parallel on the base of the calipers.

    If separate rounds, perhaps your neck tension is too high and you are getting variance in actual seating depth from brass hardness differences.
    I noticed my gauge was cutting into the bullet a little as I tried to spin it to get minimum measurement. I took my chamfer tool lightly to it to take away the sharp edge. Helped with consistency.
     
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  14. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew

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    Since its based off the 243 case with the main changes being a 31deg neck pushed forward you should be getting the same ctbo measurements that you would get with a 260
     
  15. Bc'z

    Bc'z Gold $$ Contributor

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    Just finished loading 50 of the Carolina load.
    140 grain berger vlds, sorted bbto
    2.80 coal
    2.155 cbto

    143eldx un sorted
    2.8315 coal
    2.2005 cbto

    260 rem
    130 berger ar hybrids sorted
    2.0030 cbto
    2.8105 coal.
    I put 260 up as referance only.
    Its coal is longer by .01 compared to
    140 berger

    And all these #'s only mean anything to me and my gear.
    Your comparitor might be a hornady as is mine, but there are variables in manufacturing and your finger pressure vs mine.
    Just as in bullets from lot to lot it's why we sort. Hope this helps you a little in understanding.
     
  16. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew

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    Yours is a creedmoor. Hes loading comp match
     
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  17. Bc'z

    Bc'z Gold $$ Contributor

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    Well at least the 260 data wont go to waste
     
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