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# Seating depth vs Pressure

#1
05-22-2011, 03:13 PM
 Silver Member Join Date: Jul 2010 Posts: 126
Seating depth vs Pressure

If I seat the bullets farther out toward the lands, is pressure likely to increase or decrease? I've read a lot of conflicting information.

Thanks
#2
05-22-2011, 04:29 PM
 Platinum Member Join Date: Oct 2009 Location: Thunder Basin, WY Posts: 1,787
Re: Seating depth vs Pressure

In my experience, seating bullets out to touching or jammed in the rifleing gives faster velocities (assumed to mean higher pressures). I do not own or use a strain guage so velocity and case measurements along with traditional pressure signs are all I have to go on. This also makes logical sense, because the bullets encounter resistance to movement the instant they begin to move. Whereas if they're seated back say .050", they have a chance to gain some momentum before they hit the barrel.

I believe where the conflicting information comes from; is when someone develops a max or near max load with bullets just kissing to back X distance from the rifling, and then they seat them in much deeper without changing the powder charge. In effect, decreasing the case capacity in some cases and thereby maybe increasing the pressure. I think this is more likely to happen with compressed loads or cases that are very full of powder.

Again, seating out further seems to increase pressure. This has been my experience and consequently my opinion.

If someone else has different experiences, please share.
__________________
Aim small = Miss small
SBruce

#3
05-22-2011, 04:45 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 2,830
Re: Seating depth vs Pressure

+1 SBruce

Just look at a typical 5-10thou seating adjustment. Pick up calipers, set 10thou and look at the sliver of a gap there. Now imagine how many of those slivers would fit in the full length of a case, most of which is far larger in diameter.
The percentage of volume change with deeper seating is usually very very tiny.

But timing is another matter. Powder burn timing, and peak pressure are extremely sensitive to the resistance of a bullet hitting or against the lands. But this also would vary with powder type/speed. Just as the seating volume percent change would vary with case size.
#4
05-22-2011, 06:03 PM
 Platinum Member Join Date: Oct 2007 Location: Mountians of SW NC, near Asheville Posts: 1,565
Re: Seating depth vs Pressure

Seating deeper with a handgun cartridge will increase pressures because of the small combustion chamber and very fast buring powders.

Seating deeper with a rifle cartridge decreases pressures, at least within common sense limits, because the bullets will have a running start before hitting the resistance of the rifling.
#5
05-22-2011, 06:49 PM
 Silver Member Join Date: Jul 2010 Posts: 126
Re: Seating depth vs Pressure

So, if I have a load that is very accurate but shows SLIGHT pressure signs (slightly sticky bolt lift about 50% of the time and very slight primer cratering) I could seat a little deeper and possibly lower the pressure a little?
#6
05-22-2011, 07:19 PM
 Platinum Member Join Date: Oct 2009 Location: Thunder Basin, WY Posts: 1,787
Re: Seating depth vs Pressure

Quote:
 Originally Posted by desertbull So, if I have a load that is very accurate but shows SLIGHT pressure signs (slightly sticky bolt lift about 50% of the time and very slight primer cratering) I could seat a little deeper and possibly lower the pressure a little?
In theory, yes. Though maybe not enough to change the pressure signs you're seeing, Depends on where you're seated now and how much over pressure you actually are.

Where are your bullets seated now? Jammed, Kissing, or Jumping? If not kissing then how far are they jumping or how deep are they jammed? Do they feed in magazine, are they deep enough in the case that you've got at least a 1/2 caliber down inside the neck?

Perhaps that rifle really really likes the seating depth you've already got and changing that could decrease accuracy some. But, won't know till you try it.

That is something you could easily try before dropping the charge just a hair. I've gone back later and seated bullets deeper in loaded rounds many times. Alot of times, just a 1/4 grain decrease in powder does the trick too. Changing to a cooler primer sometimes does the trick. Also, keep in mind that some rifles just plain shoot their best at or near max pressure.
__________________
Aim small = Miss small
SBruce

#7
05-22-2011, 07:29 PM
 Silver Member Join Date: Jul 2010 Posts: 126
Re: Seating depth vs Pressure

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.

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