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Pressure relative to distance from lands.

 
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Old 01-29-2010, 09:25 PM
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Pressure relative to distance from lands.

Hello, I've been wondering about this for awhile. I know that pressure is high when the bullet is jammed into the lands, and I know that when the bullet is seated deeper into the case, pressure is raised because of the decreased internal capacity of the case.

I've been curious if there is a certain sweet spot where pressure is low, and I can still load the bullet out long to save room in the case for powder. I was looking for some info on a 308 Win. I'm gonna be loading for in the spring, and I found this bit of info at 6mmbr.com.

" When choosing a load to test, keep in mind that seating depths can make a HUGE difference in pressure. Remember that seating bullets into the lands can raise pressures but moving 0.020" or more AWAY from the lands can also raise pressures, because you are reducing the effective case capacity. Many readers ask us "should I jam or jump my bullets." There is no right answer for every gun. For every guy getting great results at .015" jam, there is another guy doing well with a 0.025" jump. In general, most popular .308 match bullets (Bergers excepted) are tolerant of jump. This is certainly true of the 168 and 175 SMKs. So don't worry that you may be sacrificing a great deal of accuracy by loading to mag length, even if that puts you pretty far from the lands."


The way it looks to me is that people are seeing the sweet spot I'm looking for at the 0.020" off the lands mark.

Has anybody here experienced this??

I want to apply this to the 308 Win. by loading heavier bullets in a 30" 1-10 twist barrel. I'd like to use a powder that fills the case right up to the base of the bullet, or slightly compressed. But, I would like to know it just because, and I'd apply it to others as well.


Thanks, Eric
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Old 01-29-2010, 09:37 PM
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Re: Pressure relative to distance from lands.



I have not seen evidence that the pressure will increase by seating deeper in the case. Perhaps if you seated so deep that the ogive was past the case mouth and that would be because of mechanical obtruction.
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Old 01-29-2010, 09:45 PM
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Re: Pressure relative to distance from lands.

Even with the same powder charge weight? Seating deeper wouldn't affect pressure?
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Old 01-29-2010, 11:16 PM
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Re: Pressure relative to distance from lands.

The chart above has the same charge weight each time. The brass does not put up much resistance to the pressure and the case expands readily. What lowers the pressure is that the bullet is moving and if it hits the lands on the run then that diminishes the pressure needed for the bullet to engrave into the lands. Sort of a running start.

Barnes has some info here about 1/2 way down the newsletter

September 2007 Barnes Bullet-n Barnes Bullets

and Hornady has some info down at the bottom of this page

Internal Ballistics - Hornady Manufacturing, Inc

I have noticed the effect of seating away from the lands when loading for some 300 win mag and other calibers like 300 Wtby, 300RUM and 338RUM. If a rifle has a long throat or a short mag then it takes more powder to get velocity. The 300 win mag is a particularly good example as the distance to the lands from the case head to the lands can be the same in 2 guns but if one has a short mag then it will be slower than the one where you can load closer. Same combustion chamber size but different distance to the lands.

"Even with the same powder charge weight? Seating deeper wouldn't affect pressure?"

It will lower the pressure.
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Old 01-29-2010, 11:49 PM
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Re: Pressure relative to distance from lands.

Quote:
Originally Posted by woods View Post

"Even with the same powder charge weight? Seating deeper wouldn't affect pressure?"

It will lower the pressure.
I agree. Powder charge remains the same then the pressure and velocity will decrease with increased seating depth. At least within reason - like providing you don't pound the bullet down in the case with a sludge hammer.
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Old 01-30-2010, 07:48 AM
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Re: Pressure relative to distance from lands.

I've experienced this with a 300 Win mag in a Browning. Short mag, long throat. My velocities were lower than I expected.

I guess that was because my pressures were lower.


It goes along with the same theory of the Wby. cartridges with the freebore. Effectively decreases the pressure with a given charge, allowing a hotter charge to be loaded, giving higher velocity.


Thanks for the help.
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