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Seating depth vs Pressure

 
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  #8  
Old 05-22-2011, 06:55 PM
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Re: Seating depth vs Pressure

Quote:
Originally Posted by desertbull View Post
I'm .02 off the lands right now. I was thinking that if

7mm Rem Mag
70 grains of H1000
CCI Mag primer
162 grain AMAX
28" barrel - Brux

.75" 3 shot group at 200 yards.

Opens up to 1"-1.125" at 69.5 grains.

BTW, anyone else notice that the current Hornady manual says 68.5 grains H1000 max charge for the AMAX, yet Hodgon and Lyman both say 70 grains? I started at 67 grains and worked up in .5 grain steps. No pressure signs at all until I hit 70 grains.
They are that way with the 22-250 and H-380 also. Similar experience there.

If jumping .002 is all you've tried so far, by all means seat a few in deeper to .007 or .010 and try that before you mess with dropping the powder a 1/4 grain. Really, you could even try both scenarios side by side.
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  #9  
Old 05-22-2011, 07:08 PM
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Re: Seating depth vs Pressure

Running it through QuickLoad, I predict that with as much as 25thou deeper seating(quite a bit), you could expect a 1Kpsi/45fps drop.
No reason to do that, but the point is pressure WOULD drop, even with a big adjustment deeper than you're at.
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  #10  
Old 05-22-2011, 08:30 PM
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Re: Seating depth vs Pressure

Personal experience has led me to the same conclusions as SBruce and the rest. Seating into or very close to the lands WILL increase pressures. I experienced the same thing you are asking about in my 338 Khan a few years back.

I was seating bullets hard into the lands and like you was running right on the ragged edge of having pressure problems, hard bolt lift, lose primer pockets, etc. When I started seating the bullets .015 off the lands the pressure signs went away and I was actually able to increase the powder charge and gain significant velocity with no pressure indicators. To me, this indicates that a bullet seated very close or into the lands can create an initial pressure spike.

It is true that if you seat bullets very deep you can decrease case capacity and cause an increase in pressure, but the difference of .002 off the lands to .025 off the lands is so small a difference in case capacity to make little if any difference in pressure due to case capacity. With my Khan I had it throated long so I could seat a 300 SMK way out there and gain some case capacity. This does work, but we are talking much greater seating differences than just a few thou.
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  #11  
Old 05-24-2011, 01:04 PM
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Join Date: Jun 2007
Location: Okanogan, Wa
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Re: Seating depth vs Pressure

My recent experience with a 300RUM mirror's SBruce's explanation. I worked my load up just touching the lands to find max load with 200 gr accubonds and retumbo. I then started seating the bullets deeper to fine tume accuracy. It surprised me when I started to show higher pressure signs as I backed off the lands and increased the seating depth. Further research on the internet concurs with SBruce's explanation. I then had to reduce the powder charge. Good thread.
Thanks
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  #12  
Old 05-24-2011, 01:37 PM
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Re: Seating depth vs Pressure

Well, that clears things up.

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