Increased case capacity, decreased velocity?

Discussion in 'Reloading' started by mw185, Dec 5, 2008.

  1. mw185

    mw185 Member

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    Does increasing the capacity for powder inside the case by shooting fire formed cases and neck sizing only along with seating bullets farther out routinely require an increase in powder over the "book" value to achieve the same velocity. When loading this way I have noticed I need to go over "max" to get upper end book velocities. I am watching for pressure signs-especially bolt lift-no noticeable changes. Is this common?
     
  2. KQguy

    KQguy Well-Known Member

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    To my understanding,the case capacity does not change after firing.The case changes shape,but the capacity stay's the same.
     

  3. britz

    britz Well-Known Member

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    Your case capacity does increase slightly when fired and only neck sized. A fire formed case is blown out to the demensions of your chamber and lengthend to have the headspace of your chamber. It isn't much, but a little.

    As for the quesion about increasing powder... I'm not going to touch that one. Some rifles just won't reach the published velocities. I have a 30-06 that consistantly shoots (guessing from memory) 75 fps slower than what is published in the books. Some is due to chamber demensions and most to barrel lenght from what I understand.

    Good luck.
     
  4. lever-hed

    lever-hed Well-Known Member

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    Simple answer..NO. BUT, what are you shooting? are you cleaning your barrel?

    You shouldnt have to increase powder charge to get the same velocities, check your scale against a known weight. Make sure you are doing the same exact thing (in every sense of the word) in all your reloading procedures. Check it once, twice, three times.!!:)
     
  5. lever-hed

    lever-hed Well-Known Member

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    ps.

    How far out are you seating your bullets? if the depth of the bullet is less than the diameter, you are seating your bullets TOO FAR OUT. This is dangerous. If you are trying to reach the lands on a long chamber/throat, forget it. Double check an dlet us know.:)
     
  6. britz

    britz Well-Known Member

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    In my experience seating too far out can lead to two possible dangers (that I know of lol): one being that you are pressing on the lands which can cause a presure increase; and the other being that you can have the bullet slip out of the grip of the case during iginition allowing gasses to escape AROUND the case prior to it's being tightly compressed around the chamber sealing it.

    However, with regards to the later, I have only found that this happens in extreme conditions. I was loading a 300wsm w/ 110 gr vmax's and seating them about .1 or less on the case.... bad idea. Pulled the trigger and had black soote coat my bolt and an awfully perculure smell comming form the rifle. I deduced the problem after some conversations on this site.

    But as lever head said, in all PRACTICAL measures the case doesn't increase in size. But it does a little ;)
     
  7. boomtube

    boomtube Well-Known Member

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    "Increased case capacity, decreased velocity? "

    If the charge remains the same, yes. Simple gas law of physics; "an increased volume decreases the pressue IF the gas supply stays the same". (OK, physics majors, that's only a slight restatement of Boyle's Law to fit the situation, don't hit me for it because it's true as stated!)

    But, it really isn't because of neck sizing nor previous firing, etc. I mean, any case will almost immediately expand and conform to the limits of the chamber when fired with a full charge. Doesn't matter how small it started, the chamber is the only limit to how big the outside of a case gets.

    That leaves the thickness of the case walls as the controlling factor in internal volume. Case wall thickness varys on an individual instance, harder brass is usually thinner and vice versa. And the difference is slight.

    No matter what some folks say, a load that is truly on the safe side with hard/thin brass won't blow up any modern action if it's put into a soft/thick case. There just isn't that much difference in the internal volumes! But, there can easily be enough difference to blow up your groups if you change cases indescriminetly.
     
  8. Michael Eichele

    Michael Eichele Well-Known Member

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    In a very tight chamber a case may not expand enough to "add" any significant volume.

    In most factory rifles, chambers are cut very large. When you fire a round the factory spec'd case is now the size of the chamber. YES this isncreases the volume. Yes this (all else being equal) will cause a slight decrease of pressure AND velocity. If neck sizing only a slight increase of powder will be needed to get the load back to normal velocities. How slight depends on the case capacity AND burn rate of the load/powder combo. This is where chronographs are your best friend.

    Now, seating the bullet further out is another story. Yes seating the bullet out farther will increase volume but getting the bullet closer to the lands INCREASES pressure AND velocity. They may cancel eachother out TO A POINT, but sooner or later (most likely sooner) the pressure will increase dramatically as the bullet gets closer to the lands. It all depends on how much freebore you are starting with and how tempermental the power in question is.

    Clear as mud?

    If in doubt, leave the seating depth alone.
     
  9. mw185

    mw185 Member

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    My reason for the question is I am working up load for new 300wm(26" lilja , mauser action) .Worked up to max load (nosler data 200accubond) with R22 and H4831sc. Average vel(5 shot) with 72.5 R22 was 2836, for 4831 at 71.5 was 2808. as data said velocities should have been around 2960 and 2910 respectively(test barrel was 2" shorter at 24").Groups were better with the 4831 so I worked up to 73.5 grns with avg vel of 2844. I would love to get around 2900 but being a general follower of the experiences of those before me I question whether to pursue 2900. My hopeful theory was increasing case capacity for powder by seating out to .020 off lands at oal of 3.573 and neck sizing only was the reason for increased powder. What would you do?
     
  10. Michael Eichele

    Michael Eichele Well-Known Member

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    It is hard for me to say what I would do not being able to see the fired cases for pressure signs. I also dont know how close you are to the lands currently.

    If it were my rifle and there were NO pressure signs, I might SLOWLY work the bullets into the lands. It may be that simply moving them closer to the lands will get the 2900 mark due to the dramitic increase of pressure. PLEASE NOTE! This shuld be done slowly. Where ever the bullets are now, I would move them .010 closer to start and if ALL is well, I would move them another .010. So on and so forth. Also note that sometimes this can make a small pressure increase untill you get real close, then the pressure can really jump. If one were to work slowly up to the lands and still didnt have the desired velocity and pressure is still safe, then one could talk about adding more powder or even switching to a faster powder. When one starts to see pressure signs (ie: cratered primers, ejector marks, sticky exctraction ect....) Stopping and backing off 2 grains is a good idea. This will be a MAX load. At that point I would most likely be around the 2900+ mark.

    Hope that helps!

    Be carefull!