Seating depth vs Pressure

Discussion in 'Reloading' started by desertbull, May 22, 2011.

  1. desertbull

    desertbull Well-Known Member

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    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. SBruce

    SBruce Well-Known Member

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    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.
     

  3. Mikecr

    Mikecr Well-Known Member

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    +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. boomtube

    boomtube Well-Known Member

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    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. desertbull

    desertbull Well-Known Member

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    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. SBruce

    SBruce Well-Known Member

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    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.
     
  7. desertbull

    desertbull Well-Known Member

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    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.
     
  8. SBruce

    SBruce Well-Known Member

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    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.
     
  9. Mikecr

    Mikecr Well-Known Member

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    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.
     
  10. RDM416

    RDM416 Well-Known Member

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    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.
     
  11. birdrl

    birdrl Member

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    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
     
  12. desertbull

    desertbull Well-Known Member

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    Well, that clears things up.

    :D