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Increased case capacity, decreased velocity?

 
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  #1  
Old 12-05-2008, 04:05 PM
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Increased case capacity, decreased velocity?

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?
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  #2  
Old 12-05-2008, 06:17 PM
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Re: Increased case capacity, decreased velocity?

To my understanding,the case capacity does not change after firing.The case changes shape,but the capacity stay's the same.
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  #3  
Old 12-05-2008, 06:33 PM
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Re: Increased case capacity, decreased velocity?

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.
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Old 12-05-2008, 07:58 PM
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Re: Increased case capacity, decreased velocity?

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.!!
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Old 12-05-2008, 08:03 PM
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Re: Increased case capacity, decreased velocity?

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.
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  #6  
Old 12-05-2008, 10:35 PM
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Re: Increased case capacity, decreased velocity?

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 ;)
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  #7  
Old 12-06-2008, 09:02 PM
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Re: Increased case capacity, decreased velocity?

"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.
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