Case weight vs pressure

Discussion in 'Reloading' started by live2huntmt, Feb 14, 2014.

  1. live2huntmt

    live2huntmt Well-Known Member

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    Does case weight affect pressure? I just weight sorted my fully prepped brass and am ready to start finding max powder charges for specific powder/bullet combo's. Should I use the heavier cases or lighter cases to determine max loads? My thoughts are: Heavier cases should have less case capacity, which would result in higher pressures....is this correct?

    Thanks,
    Jeff
     
  2. MagnumManiac

    MagnumManiac Well-Known Member

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    Measure them with water, I bet there will be very little difference in actual capacity between the lighter and heavier cases.
    After you have measured/weighed them with water, let us know your results.

    Cheers.
    gun)
     
  3. Greyfox

    Greyfox Well-Known Member

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    +1. I have tried weight sorting brass and not experienced pressure differences within a specific lot due to the weight variation. I have seen particular cases produce a different accuracy result but have not been able to correlate it to weight. I don't bother weight sorting cases. I would expect that if you were able to detect a difference due to case weight variation in the same lot, the pressure of your load would be too high for practical use anyway. IMO
     
  4. jfseaman

    jfseaman Well-Known Member

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    Use the heavier case.

    Within brand, IMHO, you should not have much in the way of issues.
    Brand to brand, yes, the max load could be different.

    If you stick to 'book', you should not have much in the way of issues.

    Ok, now that the legaleeze us taken care of.

    If you are after the maximum load your action and brass will take yes, be vary careful of variance. I've had same brand variance of 1 grain H2O capacity. That is enough to be a danger. Without my notes I can remember how much brass weight difference that was but I definitely see a trend of brass weight vs H2O capacity.

    Danger aside now, seriously I notice velocity variation within a small range of water capacity. For accuracy shooting this is of course an issue. For normal hunting < 300, well I've only taken one shot so others will have to comment here.

    For long range hunting, of course it's the same as accuracy shooting, confidence of POI is very important.
     
  5. DocB

    DocB Well-Known Member

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    Just my thoughts on this...

    Whether you are determining capacity by sorting brass in the same lot by weight or determining capacity between different head stamps this is what I use as a guide.

    This is by Aaron Davidson, Gunwerks - Powders, Primers, and Long Range Shooting - by Aaron Davidson

    " A general rule of thumb for a large rifle case is +/-7 grains of brass weight means more or less case capacity equating to nearly 1 grain of powder. I have measured over 5 grains of weight difference in one bag of Winchester brass in 7mm Rem Mag. Changing a load by one grain of powder can cause a difference in velocity of 50 fps. That is well outside of our 30 fps minimum extreme spread criteria for long range precision shooting."

    Consistency is the key in case weights and in loads. Case weight differences in the same lot or by headstamp equate to case capacity differences due to the internal capacity of the case. Think of it this way... cases are designed to have the external dimensions of the case match the standard rifle chamber. The external case dimensions are pretty much fixed due to the mass manufacturing process of commercial brass cases. So... if there are weight differences within the same lot of cases it stands to reason that since the external dimensions are pretty much a fixed property of the case, then the difference must be in the thickness of the brass which, in turn, will either decrease or increase the internal case capacity of the case.

    Consistency being the mantra of long range, we want our case capacities within the same load to be as close as humanly possible. Why? Well, as described in the article, a powder weight difference of 1 grain can mean as much as a 50fps difference in your MV, more or less depending on your cartridge caliber. In this same example a variance of 30fps in MV will mean a point of impact vertical shift of 5" at 800yds and at 1000yds a vertical shift of 10" or 1 MOA. Remember this 1gr of powder weight difference was the result of a 7gr case weight difference.

    This is why most folks who sort their brass they try to keep case weight within 2.0gr.s in the same load, closer if possible.

    MV variances aside, as Mr. jfseaman cautioned, there are the ramifications of pressure variances at or near max loads to consider as well.

    So for me.. I sort within the same lot by weight and with separate brass by headstamp and weight/capacity.

    Just what I do, hope this helps.

    DocB

    Animo et fide "Courage and Faith"
     
  6. Max Heat

    Max Heat Well-Known Member

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    I'm not sure if I even qualify to post a response in this thread, but you may find this to be VERY interesting.

    Lets start out by being in agreement that the closer you are to having a 100% filled case, the more efficent, in terms of velocity vs powder charge, the round becomes. From what I have found when working at (or very close to) 100% capacity, a change of say, 1% in the case-filling VOLUME of the charge makes MORE of a difference in velocity than a 1% change in the charge's WEIGHT! I have discovered that when using cases having randomly varying weights, the velocity spread will be considerably tighter if the charged is measured by dropping a marked or scribed rod into down into the case and perfectly matching the height (relative to top of the neck, so case lengths must match) level of the charge in each case, rather than by perfectly matching the weight of each charge. Does this blow your mind? But such measurements are only accurate enough when the case is filled to top of shoulder/bottom of neck minimum. This method appears to effectively negate variations in velocity caused by case weight (and thereby internal volume) differences.

    Now that I have gone public with my theory, for which I only have a small amount of supporting data at this time, I welcome anyone to try replicating it, and either concur (agree) or digress (disagree) that this is indeed the "case"!
     
  7. DocB

    DocB Well-Known Member

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    Consider my mind blown! :)

    I can see the validity in your method. It is a very interesting technique.lightbulb

    What do you use for a rod that's small enough not to displace the volume of the case?

    DocB
     
  8. Reloader222

    Reloader222 Well-Known Member

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    So in other words if the H2O contents is 1grain heavier in one brand of brass in my .270Win say 69.0grain and the lower one is 68grain. So this implicates that I have a 1.47% less case volume in the one case. Does this now mean I must drop my load in the smaller volume case with 1.47% to get the equal pressure?
     
  9. Max Heat

    Max Heat Well-Known Member

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    The rounds were 7mm rem mag, and the rod was the one used to knock the case out of the sizing die and seat the primer, on a "Lee Loader" reloading kit for that round (referred to by Lee as the priming rod).

    The theory should apply "across the board" however, to ANY round. But the larger the diameter of the rod, without it being so tight in the neck that it's free movement might be restricted, the more accurate the volume measurement will be. It also needs to be a measurement that is "within" the neck, for maximum accuracy of the measurement. So we ARE talking no less than FULL cases, for the technique to achieve the maximun result. Filling the case is simply matter of finding exactly the right powder. Also, the finer the line is that is marked on the rod, the more accurate the measurement will be. That's why I mentioned "scribing" the rod.

    There IS a "variable" that could prove difficult to keep under control though. And that would be the "settling" factor, which obviously could have detrimental effects on the consistancy of the "true" volume of powder that is in the case. So absolute consistancy in the way each case is loaded & handled, will go a long way in assuring that the technique will work to it's full potential.

    One factor that I have given some thought to (but not tested as of yet), is how the weight of the measuring rod might affect the settling factor. The rod I was using is steel, about 5 1/2" long, and almost large enough in diameter to completely fill the ID of the case neck. I don't know if a rod that is made of a light weight material would offer more consistancy or not. It could be that the weight of the steel rod might actually HELP to equal out any case-to-case settling differences - I don't know.

    PS - I will admit that I did stumble upon this "discovery" purely by accident. If I hadn't been
    chronographing those shots, I would never have known.

    In the interest of NOT sending this thread off-topic, I have copied what has been mentioned regarding this topic to a NEW thread. Please direct any further responses to HERE:

    Charge weight vs volume, in filled cases - mind blowing discovery? - Long Range Hunting Online Magazine
     
    Last edited: Feb 17, 2014
  10. BountyHunter

    BountyHunter Well-Known Member

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    Interesting but some questions.

    1. What type chrono and is it accurate enough to determine any validity? If the distance between screens is 18 inches or less I question its ability to accurately measure with any degree of certainty

    2. When there is a volume difference, are the outsider dimensions exactly the same?

    3. What is your control/norm. SD/ES compared to your observed difference with your identified cases with volume variances?
     
  11. jfseaman

    jfseaman Well-Known Member

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    Yes. Reduce the load in the case with lower H2O volume/higher brass weight.

    BTW: I remember from my notes that it is pretty close to 7grns more brass is 1grns less water grns
     
  12. Reloader222

    Reloader222 Well-Known Member

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    Thanks jfseaman!