Need educated. Pressure, FPS, and seating depth

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by BrentM, Oct 13, 2013.

  1. BrentM

    BrentM Well-Known Member

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    Question what is the relationship between Pressure, FPS, and seating depth? Does more seating depth create more pressure or vice versa? Does seating depth have a direct impact on FPS?

    Here is my issue:

    The rifle I am shooting 6.5x284. 140 vld. 24" barrel. 56.5 grains retumbo. .100 off lands. Lapau brass. Fed 210M primer.

    There is another thread regarding this chambering that had got me to thinking, I know I know, I should not think..... but hey too late. Thinking about whether I could load to the lands and have enough bullet in the case neck for proper tension and alignment. I wanted to test and see if I could do this and gain accuracy.

    I tested some loads where the bullet was seated to the lands, in fact, .003 in the lands. I tested a 3 shot group and they all flew in the 2970 range. I am seeing faint extractor marks. Accuracy was OK, .75 at 100, the wind was horrible, gusting from every direction up to 16 mph. It blew my chrono over, the target over about 5 times, it was a pain to say the least.

    I shot my normal load that is .100 off the lands, 2900, which I expected. I am still seeing some extractor marks, little stronger, with 56.5.

    Hmmm, so I load up another group and pulled them back a little, 002 off the lands, and they flew 2950. Still pressure signs, but faint.

    OK, so I am baffled. I load up another 3, .002 from the lands and change from 210m to 215m and I am getting 2980 to 3000. .75 MOA in the crappy wind.

    Load up another 2, the last I had to test in the field, and pulled back to 56 grains, .003 into the lands, and a solid 3000 shows up on the chrono and .30 group. I wish I had a 3rd to at least get one more on paper, but I saw what I needed to at least do another field test, no pressure signs.
     
  2. Derek M.

    Derek M. Well-Known Member

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    I'm not an internal ballistics expert but I've seen pressures rise when seating deeper and also when jammed into lands. Anything in between is a toss up depending on powder charge.
     

  3. Korhil78

    Korhil78 Well-Known Member

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    Every load for my rifles that I have done with Berger bullets likes them between .005" - .030" off the lands. But my sweet spot seating depths for those rifles have been between .010" and .015". Now that is not proven science but just what I have found in my rifles.

    I never load my hunting reloads into the lands because on of these days you will go to eject a loaded round and get a chamber full of power and a stuck bullet.
     
  4. BrentM

    BrentM Well-Known Member

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    I agree and have always been concerned too. However this was a test and I don't understand the result.
     
  5. FEENIX

    FEENIX Well-Known Member

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    Hornady 9th edition reloading manual has an excellent explanation with illustrations on pages 25 and 26. I tried to scan it but not very good.

    ADDED:

    Decided to attach it anyways just in case you can decipher it at your end.

    [​IMG]
    [​IMG]
     
  6. Mikecr

    Mikecr Well-Known Member

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    I agree with Derek M
     
  7. BMF

    BMF Well-Known Member

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    In my 6.5x284, I lost about 15-25 fps every .015 I loaded away from the lands. It looks like your velocity is acting like it should. And certainly when you go with the hotter primers your velocity is going to increase. But regardless of your extractor marks on your brass, if you're using the same powder and primers and your velocity goes up or down due to seating depth, so does your pressure. They will be related.

    Pressure signs are a tricky thing, especially with Slow burning powders like retumbo. I've found that with Retumbo I was getting the "occasional flyer". When I say flyer, I mean on my chrono and paper. Even though I wasn't near the max load, sometimes I would get an unusually high or LOW ( which is odd) chrono reading.

    If your seeing ejector marks, especially on Lapua brass, that's the first sign ( not a good gauge though) of pressure. It's not necessarily a red flag point, but......you know. From there just monitor your velocity, which is a good gauge of pressure. Do at least 4 shot strings. Your ES will will start to spread when pressure is elevated.
    Brent
     
  8. BrentM

    BrentM Well-Known Member

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    OK, good info guys. Thanks BMF for the feedback. I am well within the zone of the berger manual and keep thinking I have issues with pressure signs. The biggest issue I had was at 57.5 grains I was getting the occasional flyer. I reduced the charge in .5 increments to test accuracy at .75 off the lands, my normal load. I found 57 strung vertically and 56.5 was solid .25 MOA with no flyers. I then played with seating depth and found a little more accuracy at .100 off the lands and left it alone.

    I just thought it was strange the speed would go considerably up and yet the case is not indicating the pressure has increased on the case head as much.

    I did find that the ES was much better with bullet seated to the lands vs at .100. In fact I had 2 in my test that flew 2953 and 2954. In another test they flew 3000 and 2999.

    Next time out I have more time to tinker and record results. This was just a test I wanted to try while dialing one of the kids 7mm-08 for the up coming hunt.
     
  9. BMF

    BMF Well-Known Member

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    This stuff will drive you bonkers sometimes. I feel your pain, I've been there. In the grand scheme of things, its not that big of a deal, especially in your situation because it sounds like you've got a good shooter. But it sounds like you're like me and its hard to let something go that doesn't quite add up. Good luck
    Brent
     
  10. BrentM

    BrentM Well-Known Member

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    Remember this is in some frustrating wind so grouping is really unkown, could be error to the good or bad.
     

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