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Seating depth and pressure

 
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  #1  
Old 10-02-2007, 04:52 PM
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Seating depth and pressure

338/ 300 Ultra + 101 grains US869 + Federal 215 + 300 smk (3.830 OAL)= extractor mark.

338/ 300 Ultra + 102 grains US869 + Federal 215 + 300 smk (3.930 OAL)= no pressure signs.

It appears that seating a little deeper makes a lot of pressure difference. Is this consistent with anyone else's experiences? Is there another variable I may be overlooking? These were both 3 shot groups with Remington brass. One round was fired with 102 grains in the first series and it left a shiny, but not raised extractor mark.
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Old 10-02-2007, 05:49 PM
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I don't have extinsive testing experience with this but I do notice presure increases when you get close to the lands.
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Old 10-02-2007, 06:54 PM
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britz,
That's what I thought, but I experienced the opposite...of course I was still .020 off the lands with the 102 grain load that showed no signs of excessive pressure. In fact, it was even a little cooler on the day that I shot the loads with extractor marks. Go figure.

Thanks for the input.
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Old 10-02-2007, 07:31 PM
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Are you compressing loads? That may impact the pressure as well. Also, Are you using the same lot of brass.
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Old 10-02-2007, 08:23 PM
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Still have shaking room. Same lot of brass.

I've heard that slow burning powders and deep seated bullets have the potential to cause pressure issues because the primer's ignition can actually shove the bullet to the lands and momentarily stop its travel before the powder sufficiently ignites, causing a pressure spike in the chamber before the bullet breaks inertia and starts down the tube. Sounds possible, but could be an "old reloaders tale" too. I've never seen any real evidence that this is possible.
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Old 10-03-2007, 06:55 PM
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crittergitter,
When you got the ejector mark on the case head, was this the first firing for the brass? Any oil/cleaning solvent left in the chamber by any chance?

I have seen the first or second firings on brass leave ejector marks numerous times. I normally take a single cut file and simply shave it off never to have it appear again. So you have to be careful using that mark as a true indicator of pressure. Botton line when using more powder you have more pressure with all else equal. The fact that you were still .020" off the lands even with the COL probably didn't make a big enough difference in pressure to even measure. Now if you were touching the lands and moved it that far.. that would be a different story.

I'm curious about the answer to my first couple of questions though. That is where I would look first.

Steve
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Old 10-03-2007, 07:57 PM
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Crittergetter,

A little different cartridge but same powder, powder weight and bullet.

338 RUM 300 SMK 101gr US-869. 2732FPS w/26" Sporter Barrel SD= 0 ES=2 COL=3.818" Length from Base to Ogive = 3.017"
No pressure signs whatever except a strong desire to FL resize after a few shot through a case. I am satisfied w/the velocity and accuracy to leave things there.

Your chamber and barrel may be affecting things also for comparison purposes. I'm assuming both are custom which is a complete different situation than my factory chamber and barrel. However, I did to a Tubbs Final Finnish procedure on it when new.

The greater the case volume the lower the pressure and the slower the MV. The difference between FL sizing and Neck Sizing in the above load is a reduction of 17 FPS for the neck sized shots. The same should hold true in your rifle even if the bullet was seated into the rifling.

Your rifle, if sporting a longer barrel, should achieve another 100 to 150 FPS over the RUM. At 101/102 grains and the 300 SMK you're just loafing along, I'd think.

Quote:
Originally Posted by crittergitter View Post
338/ 300 Ultra + 101 grains US869 + Federal 215 + 300 smk (3.830 OAL)= extractor mark.

338/ 300 Ultra + 102 grains US869 + Federal 215 + 300 smk (3.930 OAL)= no pressure signs.

It appears that seating a little deeper makes a lot of pressure difference. Is this consistent with anyone else's experiences? Is there another variable I may be overlooking? These were both 3 shot groups with Remington brass. One round was fired with 102 grains in the first series and it left a shiny, but not raised extractor mark.
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