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Increased case capacity, decreased velocity?

 
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Old 12-08-2008, 08:09 PM
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Re: Increased case capacity, decreased velocity?

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.
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Old 12-09-2008, 10:51 AM
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Re: Increased case capacity, decreased velocity?

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?
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Old 12-09-2008, 10:54 PM
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Re: Increased case capacity, decreased velocity?

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!
__________________
Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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