Seating depth and pressure

Discussion in 'Reloading' started by crittergitter, Oct 2, 2007.

  1. crittergitter

    crittergitter Well-Known Member

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    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.
     
  2. britz

    britz Well-Known Member

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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.
     

  3. crittergitter

    crittergitter Well-Known Member

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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.
     
  4. britz

    britz Well-Known Member

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    Are you compressing loads? That may impact the pressure as well. Also, Are you using the same lot of brass.
     
  5. crittergitter

    crittergitter Well-Known Member

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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.
     
  6. Steve Shelp

    Steve Shelp Well-Known Member

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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
     
  7. royinidaho

    royinidaho Writers Guild

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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.

     
  8. Stonewall2

    Stonewall2 Well-Known Member

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    crittergitter
    Check the headspace of fired and unfired brass with a shoulder gauge or Precision mike.
    If the case head has enough room "to get a run at the bolt face" it will leave an ejector mark on the brass -rather like the way the cases are made. I have seen this firing new Weatherby brass in a factory .30-378 chamber that had + .006 " clearance on the belt before firing.

    Clean your chamber with rubbing achool on a mop before firing. Wipe the cases down also and put greese on the back of your lugs.
    Now tell us what happened.
    Glenn
     
  9. crittergitter

    crittergitter Well-Known Member

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    Wow, lots of suggestions to reply to! I seriously doubt that the cases or chamber had any friction reducing contaminates on them. I've been paranoid about that after seeing smeared case heads and disintegrated primers result from someone moly coating their chamber (not me). Nevertheless, that is the most likely variable that I would have overlooked. There was a thorough cleaning before and after each group that was followed by an oil patch and a dry patch. I generally swab the chamber with alcohol before hitting the range, but I may not have fully stripped the chamber of oil this time. I'll give extra attention to that step next time out.

    Cases are new, and I am bumping the shoulder with the die because factory inconsistency within the lot is enough to prevent a case from chambering without excessive force every now and then. I sized the second set of cases on a different day, so I suspect that it is possible that I might have had the die set too deep with the first round, although I don't think so. I cannot measure any difference in the headspace on a newly resized case and a fireformed case, and I have not adjusted the depth of the die. I did remove it from the press and reinstall it.

    I'll post again after my next outing. If I have a positive experience, I'll assume that there was some cleaning error or other fluke variable that I overlooked. Thanks for the advise.
     
  10. crittergitter

    crittergitter Well-Known Member

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    Update on load development for the 338/ 300 RUM. I have worked up to 105 grains of US 869 and have barely noticeable (in the old days I'd still be working up) pressure signs. The 300 SMK is cruising at 2957 fps and fired a 3 shot, .6" group at 100 yards on a squeaky clean bbl. The second 2 shots punched the same hole unmeasurably close. ES is + or - 20 fps. I think I'll stick with this load for a while.