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Thread: .308, one inch, one grain

  1. #1

    .308, one inch, one grain

    G David Tubb reckons that increasing the powder charge by 1 gain will return a 1" higher POI at 100yds. A couple of people I shoot with treat this as a near certainty.

    However, while developing loads with my Tikka T3 20" barrel .308 I noticed that changing from 41.5 to 42.5gr of N140 with a 155gr SMK didn't move the POI, it only tightened the group.

    How can this be?

    A clever chap suggested that the barrel lenght means the extra bit of powder isn't being burnt efficiently - a diminishing return if you will.

    Opinions?

  2. #2
    I once saw a target the european bench rest champion had shot whilest practicing and as he loading the bullets with extra grains the bullets went around in a semi circle right handed with the 5 shot group of about 1 inch with all the bullets about 3mm apart
    AT THE AGE OF 50 I DECIDED I WAS GOING TO GROW OLD F***ING DISGRACEFULLY

  3. #3
    Makes sense too, more velocity, more spin. Still makes mine a mystery; I would have expected the POI to move somewhere!

  4. #4
    I have .308 T3 away getting cut to 20 inches and thread cut at the moment. I would be interested to hear how you got on with your load development.

    Regards

    Dave

  5. #5
    The only certainty that I know when re-loading is that every rifle and every barrel is different .

  6. #6
    I got about a 1/2" at 100yds on the first batch. Can't really moan about that so I'll probably stick with it. Next time I go (Sat) I'll chrono a few rounds of both powder weights. If there isn't much velocity difference then my mate is probably right, not all the powder is being burnt.

  7. #7
    Quote Originally Posted by Milligan View Post
    I got about a 1/2" at 100yds on the first batch. Can't really moan about that so I'll probably stick with it. Next time I go (Sat) I'll chrono a few rounds of both powder weights. If there isn't much velocity difference then my mate is probably right, not all the powder is being burnt.
    This is a small point, but unless you're using a powder that is really inappropriately slow for the case you're using, your powder will be burnt within a few inches of the chamber. The expanding gasses, however, may need a longer barrel to be utilized. Some people contend that for practical purposes, this is really the same thing: But from an internal ballistics standpoint, it is a significant point.~Muir
    Last edited by Muir; 19-05-2010 at 01:32.

  8. #8
    M. Can you post the link to David Tubb's article that talks about this effect? I would be interested to read it. Thanks JCS

  9. #9
    Quote Originally Posted by jcampbellsmith View Post
    M. Can you post the link to David Tubb's article that talks about this effect? I would be interested to read it. Thanks JCS
    Afraid not, it's on a DVD I borrowed from a mate. I'll google it and see if I can find anything.

    Muir, you make a good point. I hadn't really thought of that. It could be that the extra gas is just toasting the inside of the moderator...
    I am using N140, so not an excessively slow burner.

  10. #10
    The trouble with using the 1", 1grn thing as a rule of thumb is that it doesn't work. Within reason my Sauer 202 will put most of it's bullets into the same 2-3" group during load development, and I usually do my trial loads in 1 grain steps.
    With a standard charge through a standard length barrel you will not get unburnt powder using N140.
    You can't say muntjac without saying, Mmmmmm.

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