Opinions (or facts) about air space inside a loaded case.

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Michael Eichele, Nov 27, 2006.

  1. Michael Eichele

    Michael Eichele Well-Known Member

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    I am looking for opinions or facts about a healthy amount of air space in loaded rounds. Healthy meaning 89 grains of powder in a 300 RUM case with a 180 grain bullet when the capacity is about 105 grains with the same bullet.

    Does this have much of an affect on extreem spread? Does it increase the temperature senitivity of a given powder?

    Any thoughts, experiances or opinions are welcome.
     
  2. CatShooter

    CatShooter Well-Known Member

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    I have heard many people "create" their own theories on this over the years, but for every theory there are 100 exceptions.

    The fact that Charlie had one rifle that did "whatever" is not justification for Charlie to press his case for his theory.

    I have seen loads that had 20% air space with single digit ES and SD... and I have seen loads that were compressed, with single digit ES and SD.

    Small cases and belted Magnum cases... all the same. Each rifle and load is a new "adventure" in loading /ubbthreads/images/graemlins/wink.gif

    So the theories are just fluff on this one.

    The "air" itself, has no effect on the burning of the powder.

    .
     

  3. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

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    From what I have found with low load density situations is that you will in many cases get velocity spread increases from this.

    The reason, position of the powder in the case in relation to the primer flash. I have actually tested this in my 270 Allen Magnum with a 140 gr bullet and WC872 powder. The load I was using was about a about an 85% load density.

    TO start the test I took five rounds and fired then normally and collected the velocity data. They averaged 3210 fps with an Extreme spread of 38 fps. To be honest, this is not bad with this large of a case capacity and this powder.

    Next test was to position each cartridge bullet up and carefully chamber the round making sure the bullet never got below level. This made sure the powder was back against the flash hole.

    Averge velocity for five shot was 3225 fps with an extreme spread of 18 fps. Pretty noticable drop in extreme spread. Keep in mind, these were the same exact loads as the first test all loaded in the same batch of ammo.

    Next test was to position the cartridges bullet down and make sure that the powder was against the bullet when fired and the air space at the flash hole.

    Interestingly enough, the average velocity was 3268 fps, quite a bit higher then the other loads of the same lot. Extreme spread also increased to 77 fps.

    You can take this for what you will, only firing five shots does not make for completely convincing data but it did show a pattern.

    also remember this was with very slow burning ball powders. I have done similiar tests using stick powder and did not see hardly any difference in velocity or extreme spread with the easier to ignite stick powders.

    That said, it is my opinion that if you get the correct powder for a certain combination of bullet weight, bore diameter and case volume, you will be in the +90% load density range.

    Obviously lower load densities can be very accurate and consistant but they can be more finicky compared to higher load densities in my opinion from what I am seeing with my Allen Magnums. Now these are more extreme then most rounds so that may well be the case. Everything is magnified using these rounds to some degree so its easier to see things that may only be slight in a conventional chambering.

    One other point, in extreme cold temps, you are much more likely to see a hangfire with a low load density then you would with a +90% load density, even if the powder burn rate is compatable with the load you are using. By extreme cold I mean below 0 degrees F.

    Good question.

    Kirby Allen(50)
     
  4. zupatun

    zupatun Well-Known Member

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    When I'm loading 165gr IB's with 44.5gr of Varget or more in my .308 I always hear the load "crunch" a little. I'm assuming I haven't packed down the Varget and I'm just squashing the load a little. I should be able to easily put up to 46gr of Varget in a 3.8Win case with 165gr bullet...no reason to be concerned I imagine...have only been handloading for a year.
     
  5. Michael Eichele

    Michael Eichele Well-Known Member

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    Thanks for all the good posts.

    Fifty,

    That is some good info. Your tests are similar to some I am planning on doing myself in the near future. I will let you know what I experiance with RL-25 and about a 90% +/- load density.

    [ QUOTE ]
    also remember this was with very slow burning ball powders. I have done similiar tests using stick powder and did not see hardly any difference in velocity or extreme spread with the easier to ignite stick powders.


    [/ QUOTE ]

    When you say stick powders, I assume you are talking about powders like RL-25, H50 BMG, Retumbo ect... Am I right?
     
  6. goodgrouper

    goodgrouper Well-Known Member

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    My own experiences show that low deviations can be attained with load densities under 85% and can shoot quite well. However, it seems that low deviations that have a density of 90% or greater usually KEEP their low deviations low in circumstances pertaining to differing hunting situations related to angle of shots and differing climatic conditions.

    Powder positioning in the case before the primer flash hits it does affect how the powder burns to some degree. If you have a density of 70% and take a 25 degree downhill shot, you can bet the progression of the powder changing to a gas will not be the same as case full of powder under the same shooting condition. The degree of change will be different for each powder and primer and as Catshooter already stated, each individual gun is it's own animal.

    Only individual testing of each gun will show how much difference the density will make, but just to eliminate having to test every little thing in every gun, I try to get above 95% density just so I can count on that load even if I have to take a steep downhill shot. If the gun likes a load that has a density under 85%, then I will go test it on angled shots in colder and warmer weather to see if there will be a surprise!
     
  7. Fiftydriver

    Fiftydriver <strong>Official LRH Sponsor</strong>

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    Meichele,

    Yep, your on the money. I would be suprised if you see any measureable differences with a 90% load density with the Reloader powder but it will be an interesting test to see the results on.

    Kirby Allen(50)