OCW, OBT, Seating depth, pressure

Discussion in 'Reloading Forum (All Calibers)' started by grovey, Jul 1, 2019.

  1. grovey

    grovey Silver $$ Contributor

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    I've been chasing my tail this yr doing load development, and while I'm just about there... the one aspect of the tail chasing has me thinking/wondering.

    When I find a charge and seating depth that delivers,or borderline delivers the level of accuracy I need... I try to pin point it by adjusting seating depths and charge. What I've found is when I'm this close I can up the charge a tenth, and set the bullet out another 3 to 5 tenths, or drop it a tenth and seat deeper and get results that have me wondering if I'm heading the right direction.

    By doing this I assume I'm mimicking the pressure I need to squeeze out the best load? What should I attribute these results to the most? Is it obt,ocw, seating depths, pressure, or all of the above?
     
  2. dsculley

    dsculley

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    On your OCW, are you picking the "best/smallest group" or the middle of a set of groups that have the same center on the target?
     
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  3. grovey

    grovey Silver $$ Contributor

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    I'm basically trying to make sense of how changing seating depth, and charge as mentioned above appears to keep me in a sweet spot. I assume it's producing a like pressure/wave which has the bullet leaving the bbl at the perfect time. That makes sense to me, but it skews the thought of one perfect seating depth in my mind. As I said,I'm pretty sure I'm where I want to be, but this had me thinking.
    But to answer your question.... Both. A small group always gets my attention, but I've been trying to work on the better looking groups with the same water line.
     
  4. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    I'm not sure anyone has a detailed explanation of exactly what is happening when seating depth is used to tune in grouping. I've heard a lot of different theories over they years as to different things it might be...such as barrel timing or entry into the lands/bullet engravement. Certainly there are arguments that seating depth could affect both, as well as arguments that neither one is the sole factor affected by seating depth.

    One thing to remember about the notion that seating depth affects barrel timing/harmonics is that the bullet is moving infinitely slow in the first few instants after the weapon is fired. Because of that, even the extremely small [relative to barrel overall length] increments we use to test seating depth may have a profound affect on barrel timing.

    As a general rule, I have not placed a great deal of emphasis on trying to understand exactly what we're accomplishing with seating depth optimization for the purpose of trying to predict how to optimize it. It is far more easy [and reliable] to directly test seating depth and let the rifle tell us where it wants a certain bullet seated than to try and guess what might happen to seating depth optima as charge weight/velocity are altered. For example, changing seating depth for jumped bullets by as much as .010" to .015" in either direction will generally cause a change in velocity due to altering the internal case volume by an amount that cannot be reliably detected by most chronographs...i.e. the change in velocity will be less than the ES/SD. So changing charge weight by even 0.1 to 0.2 gr will generally cause a more noticeable change in velocity than a large change in seating depth in that you can usually detect a significant change in velocity for that change in charge weight. Nonetheless, a change in seating depth of even .001" to .002" can have a huge impact on grouping. More often than not, I find the seating depth optimum for a given bullet doesn't change much if I adjust the charge weight very slightly in either direction, or the velocity is changed due to temperature differences on different days. However, I shoot F-Class, where it is not necessary to have a load that shoots 0.1 MOA (or less) to win. The people that can probably give you the best anecdotal information about this topic are the short range BR shooters, whom measure and change such parameters regularly, and record the results religiously. Perhaps a few will chime in on this topic.

    Anyhow, if you are wanting to see any relationship between velocity and seating depth more clearly, try setting up a charge weight versus seating depth array, where you test increasing charge weight (vertical dimension) at several incremental seating depths each (horizontal dimension). That way, you can observe how both parameters affect grouping on the same target. For statistical reasons, you might actually want to shoot a couple identical targets.
     
  5. mikecr

    mikecr

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    I believe land relationship (set w/seating depth) offers conditions where the bullet engraves consistently and with least rattling about, or just opposite. That is, it's clearing or contributing to a quality of engagement with the bore.
    Folks may suggest then that seating into the lands (ITL) should always be best, but that jacks up starting pressures as well (two changes at once), which is often not leading to a best load for hunting capacity cartridges. While seating is vital to best grouping, a very coarse adjustment to results, I'm certain that seating as 'tuning' fails tests.

    OBT is merely another condition improved or degraded, but through powder adjustment. We're ensuring, through timing of travel, that a bullet is not muzzle releasing while the bore is expanded due to cycles of vibration.
    Again, it's not really tuning, and not correlating with the function of an actual tuner, which affects an attribute of barrel movements.

    There are other conditions to clear,, primer selection & striking,, gun balance & rest behaviors, etc.

    OCW seems only an endeavor for most forgiving powder load. It is tuning -for that, and not tightest grouping.
    To find a place where a relatively wider range of powder burns with a flatter affect to MV.
    I imagine it's possible to have actual best accuracy with an OCW load, but rare for most of us without a tuner.
    And then:
    There is another type of pressure tuning with no name ever given to it. This is taking small underbore cartridges deep enough into diminished returns with extreme pressure loading. Diminished returns = diminished variances of it (a different plateau). Here, seating ITL is useful enough to gain both high starting pressures and group shaping. That's your competitive 6PPC.

    Incremental load development (ILD), ladder or otherwise, is pretty much all inclusive even while we don't know specifically what is contributing to what. Seems prudent to me to clear both seating and OBT potentials prior to engaging in ILD, even with a tuner. Like powder itself, neck tension is a fine adjustment here.
     
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  6. damoncali

    damoncali Bullet Maker Silver $$ Contributor

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    If you can figure out why seating depth matters so much, I'll pay you to tell me. It's my opinion that it's a vibration thing - as you move in/out you can see the groups change, just like when using a tuner. Why such small changes matter is not something I can explain. It's hard to quantify what happens early in the firing process. Even programs like QuickLOAD ignore ignition.

    I've also wondered if it's a dynamic thing, where the bullet tips on its way into the bore in an oscillating manner. But some chambers are really tight on the freebore, and we still see seating depth matter, so that's probably not it.

    Pressure certainly changes with seating depth. But again, how much exactly is a bit of a mystery. It likely has to do with starting pressure (the amount of force required to engrave the bullet) and other ignition issues. The more I learn about shooting small groups, the more I think ignition is a key. And the more I realize how little we know about it.

    Bottom line is that someone's guess on how this works might well be right, but proving that is not at all easy. It could be a combination of mechanisms. We just don't know.

    As for the rest, OCW is just a load development methodology - nothing magical there. It's trial and error done in a reasonable way. OBT is not good theory, so it's not worth getting into. Some people find that the nodes it predicts are good, but I can't see how that can be anything but coincidence. I've gotten to the point where I can almost always find a good load by picking a high load density powder with good velocity and max pressure according to QuickLOAD. At that point, dialing in seating depth (or using a tuner) always seems to find me acceptably small groups. I rarely even test charge weights or powders anymore.

    (Caveat: I do not shoot benchrest competitively - they have things dialed in very tightly because they've been shooting the same stuff for decades with minor tweaks along the way, so they have different parameters to work with. Much of what can be tried already has been, so they can fiddle with stuff in the weeds in search of another few thous of group size.)
     
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  7. dmoran

    dmoran Silver $$ Contributor

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    Your inputs to Benchrest are laughable, but like much of your input, speculation and assumption run rapid, the way I see it.
    But hey... it's obvious you have Quickload in your back pocket, to base input from often enough !.!.!
     
    Last edited: Jul 2, 2019
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  8. damoncali

    damoncali Bullet Maker Silver $$ Contributor

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    I think you misunderstand me, which is understandable since I'm not always clear (whatcha gonna do? I am what I am). What I mean is that "QuickLOAD and go" isn't going to hack in in benchrest. I've found it does work for rifles with lesser accuracy requirements (like my TR rifle).

    At least, when I shoot my BR setup, it's not that easy, and I don't even compete in BR. I just do it for fun. Benchrest is obscure and arcane, I don't pretend to know even half of it. What I do know is that that the 6PPC (and even others like the .30BR) has been flogged nearly to death from all directions, and QuickLOAD isn't going to help anyone out there.
     
  9. dmoran

    dmoran Silver $$ Contributor

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    Better to not generalize Benchrest, especially without having any experience with it.

    If your not testing powder types and the charges by relying on QL for those critical aspects, in my experience, your likely leaving a lot on the table (more so yet, for LR and extended distances). But it sure must make tuning easy, when assuming so much !.!.!
     
    Last edited: Jul 2, 2019
  10. ckaberna

    ckaberna Gold $$ Contributor

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    Very well written. Thanks for your time.
     
  11. damoncali

    damoncali Bullet Maker Silver $$ Contributor

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    We absolutely agree - both that my benchrest knowledge is limited to my personal experimentation (way short of a serious competitor), and that I believe you must test powder charge weights (among other things) to get respectable groups at that level of accuracy. No disagreement there at all. That's what I meant initially - that there is a huge amount of detailed practice and experimentation that I know goes into it, and you cannot use QuickLOAD shortcuts once you hit a certain level of precision.

    But... what I'm saying is that if you can live with slightly worse accuracy, you really don't need to test powder charges much. Are you leaving something on the table? Yes. Absolutely. But not much in my experience when it comes to F Class accuracy requirements - a 1 MOA ten ring is pretty big. Having a 1/4 minute gun vs a 3/8 minute gun isn't going to matter, and most guys aren't going to be able to keep 1/4 MOA in tune over a long match anyhow. Guys who think they have 1/4 MOA guns are not rare. Guys who actually do *are* rare. I've managed to shoot X counts in the mid-teens doing this (on calm days that really let you see a rifle's potential). Spending a lot more time and money testing might have get me another X or two. It's not going to make a 9 into a 10. Different games, different techniques.

    I know this amounts to heresy. But I gave up on testing powder except as a safety measure, and it hasn't held me back at all. It works *if* you choose wisely. 99% of the time, a stout load of whatever everyone else is using does the trick.
     
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  12. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    Maybe so...but seems nearly microscopic when the wind is blowing. ;)
     
  13. CharlieNC

    CharlieNC Gold $$ Contributor

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    And when the mirage is boiling!
     
  14. damoncali

    damoncali Bullet Maker Silver $$ Contributor

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    Ain't that the truth.
     
  15. murray brook

    murray brook Silver $$ Contributor

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