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Thread: .20 tac disappointing velocity

  1. #1

    .20 tac disappointing velocity

    Hi, I wonder could I ask some advice.

    I have worked up a few good accuracy loads for my .20 tac using ramshot xterminator. However, when I came to chrono today it looks like I'm not even coming close to the velocities quoted in the load date.

    Load is as follows:

    cases: reformed .223 lapua match
    primers: cci 450
    powder: 24.2gns ramshot xterminator
    bullet: 40gn v mamax seated to book col

    barrel is 24 inc Pac nor
    now the manual reckons for this bullet powder weight combo I should be around 3600 fps but my chrono reads 3450 which the book reckons I should be seeing from just 22.2 grains! No pressure signs to speak of, just the tiniest bit of cratering on the primers but this load is still well short of max!


    Anyone suggest a possible cause? I'm not using the same primers but surely it couldn't make that much difference?

    thanks for any advice!

  2. #2
    Quote Originally Posted by hcm1 View Post
    Hi, I wonder could I ask some advice.

    I have worked up a few good accuracy loads for my .20 tac using ramshot xterminator. However, when I came to chrono today it looks like I'm not even coming close to the velocities quoted in the load date.

    Load is as follows:

    cases: reformed .223 lapua match
    primers: cci 450
    powder: 24.2gns ramshot xterminator
    bullet: 40gn v mamax seated to book col

    barrel is 24 inc Pac nor
    now the manual reckons for this bullet powder weight combo I should be around 3600 fps but my chrono reads 3450 which the book reckons I should be seeing from just 22.2 grains! No pressure signs to speak of, just the tiniest bit of cratering on the primers but this load is still well short of max!


    Anyone suggest a possible cause? I'm not using the same primers but surely it couldn't make that much difference?

    thanks for any advice!
    Well. Reloading data is just a guide, not an absolute, but I can see where you would be disappointed. Neck tension and all is good?? Primers can make a difference. Switch to a non-magnum primer and see if that helps. You never know....~Muir
    Last edited by Muir; 30-06-2013 at 04:53.

  3. #3
    hi Muir,

    thanks for the reply, would velocity increase or decrease with low neck tension? the cases im using are 4th or 5th fired so maybe they are getting a bit hard? that would lead to decreased neck tension i assume? ill try a switch to rem 7 1/2s and see. maybe try some new cases?

    im not looking for ridiculous speeds, accuracy is more important but i dont see why it is taking me a whole 2 grains of powder more to achieve close to book velocity.

    one other thing, the leade in my rifle is very long, i cant seat anywhere near the lands as i would only have maybe 3mm of the bullet in the case, can this be a factor?

    ​thanks for taking the time to reply!

  4. #4
    You're only 4% lower in the velocity than quoted figure.

    Chronos are not always accurate, temperature plays a role, powder differs a little from lot to lot etc. This in addition to your observations of primers and leade (longer leade can considerably cut down the pressure spike and thus slow down the highly progressive powder burn rate).

  5. #5
    hi mate, thanks for the reply, i wouldnt mind if i was just 4% low but im 4% low using 2 grains more powder than the book reckons i would need! i accept ill not achieve book figures but surely this is way off! could the leade make this much difference?

    ​cheers

  6. #6
    I was after a Twenty tac, but couldn't persuade Cheshire, if I had , I was going to work up a load from Benchmark. Steve.
    (The Unspeakable In Pursuit Of The Uneatable.) " If I can help, I will help!." Former S.A.C.S. member!

  7. #7
    hmm just had another look at the numbers and there is only 280 fps difference between start and max velocities, maybe i am obsessing a bit much, i just hope i can push the 39gn blitzkings fast enough to make use of the bc.

    thanks for taking the time to reply.

  8. #8
    steve,

    what was cheshires problem with the .20?
    maybe ill give benchmark a go if i can find some near me!

    ​cheers

  9. #9
    If I understood correctly, book says:

    - 22.2gr gives 3450fps
    - 24.2gr gives 3600fps

    This (getting only 4% more speed with 9% more powder) is just a symptom of the progressive burn rate. The min load (or near) burns at only moderate rate. The heftier the load, the more aggressive burn.

    In the case of long leade, the initial case volume will easily grow because of bullet movement. Also when bullet is engaging the rifling, the resistance could be lower (compared to e.g. jamming the bullet into rifling when loading the rifle). This lowers the pressure spike which in turn prevents the higher burn rate from occuring. So it's possible your higher 24.2 load will behave like min load from the book.

    Since you didn't see pressure signs (I presume mild cratering was due to generous firing pin hole in factory gun) I wouldn't be too worried. You could doublecheck your powder scale and also chrono some factory loads or loads with different powder to see if they're also on the low side.

  10. #10
    Are you relying on Todd Kindler's data?
    Knowledgable and helpful as he is, I genuinely believe that his chronograph is broken!
    ​My tac runs 40 grain V Max at the same speed as yours.

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