Seating depth mystery!

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by elkaholic, May 26, 2010.

  1. elkaholic

    elkaholic Well-Known Member

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    I was shooting my 6.5 Sherman today with the intention of testing accuracy with different seating depths. I have been seating the 140 Bergers about .010" into the lands with good results but was curious to see what the results would be if I went the other way with large jumps. I was shooting 63 grs. of retumbo with the following seating depths. (1) .010" into lands. (2).040" off of lands (3) .100" off of lands. (1) three shots averaged 3125'. (2) 3 shots averaged 3172 (3) 3 shots averaged 3146'. I thought this was really odd since the spreads within groups averaged 10-15' and the second group seated .040" off the lands was considerably higher than either end? I decided that it might be a brass problem so I reloaded the same three groups only switched brass between seating groups. The temp had cooled off so velocities were a little lower but the results for seating depths remained consistent. Groups 1 and 3 came closer together for avg. but group 2 (.040" off lands) remained about 40' faster on avg. I didn't get around to finishing my original goal because I am still scratching my head as to how this could be? Does anyone have an explanation for this.......Rich
     
  2. royinidaho

    royinidaho Writers Guild

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    It would be reasonable, given the same powder charges, to expect that the shortest COL would give the highest velocity.

    However, why the 0.04" jump gives the higher velocity is a bit of a puzzle.

    I could send you an RSI pressure lab if you had a laptop to connect it too. The RSI is pretty revealing. When it comes to pressure curves/spikes/multiple peaks etc.

    It's probably the "Sherman" case design. :D
     

  3. ICANHITHIMMAN

    ICANHITHIMMAN Well-Known Member

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    I think this is just the VLD's Eric recomends a seating depth test. Depending on which bullet you use the Hunting or Target you have to vary your seating depths.

    Not sure about the pressure spike either. Was it formed brass?

    Jon
     
  4. J E Custom

    J E Custom Well-Known Member

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    I have found that more free bore normally increases velocity and allows the use of more powder
    without pressure.(That's why Weatherbys are higher velocity at safe pressures)

    The Weatherby factory ammo should not be shot in a chamber with very little free bore because
    it will normally blow the primers.

    Seating close to the lands will force one to back off the powder charge a little.

    J E CUSTOM
     
  5. elkaholic

    elkaholic Well-Known Member

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    These were identical loads with varying seating depths. Why would the middle one give the highest velocity?
     
  6. Mikecr

    Mikecr Well-Known Member

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    It could be where peak pressure is occuring in the powder's burn curve.
    Could probably see it well with a strain gage.

    At 10thou, there is more case capacity per powder, less efficient peaking ->lower pressure
    At 40thou, there may be very efficient peaking of the burn curve, with slightly reduced case capacity
    At 100thou, there is less case capacity for higher pressure, but again inefficient peaking of the burn curve, because the bullet hesitates little through land engravement(no peak).
    All of this with YOUR exact conditions, and not applying to anyone else really(why would it?).
    This is why you try a few powders in load development.
     
  7. elkaholic

    elkaholic Well-Known Member

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    I did notice that the case necks were pretty smokey with the retumbo........
     
  8. royinidaho

    royinidaho Writers Guild

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    Rich,

    The reason I mentioned the RSI pressure lab is that it revealed, in a 338 RUM, with certain powders and certain load set ups a secondary pressure spike would be created somewhat prior to the bullet exiting the muzzle.

    The secondary pressure spike (if that's proper terminology) was repeatable.

    I don't have any written record of it though. After discussing with the fella at RSI he said the spike had been observed by others in other calibers also.

    I don't recall measuring MV differences. I should improve my record keeping a bit.
     
  9. elkaholic

    elkaholic Well-Known Member

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    Thanks Roy, that is quite interesting. I wouldn't have been so surprised if the load engaging the lands had higher pressure (velocity) or even if the load seated well into the case had higher pressure because of lowering the case capacity "at least until the bullet entered the lands". The big surprise was that it happened to the extent it did and right in the middle load. There are a LOT of dynamics going on that I guess we don't fully understand......Rich