Measuring OAL in precision ammo...

Discussion in 'Reloading' started by richardca99, Mar 14, 2013.

  1. richardca99

    richardca99 Member

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    I've often wondered how others do this -- particularly those who are building precision ammo. I'm talking about BR quality ammo (and yes, I have a rifle that will allow me to see the variance on paper).

    When I measure loaded ammo with my Starrett calipers, clamp-on comparator, and comparator base, my setup allows for very repeatable, precise readings. I use Redding competition micrometer seating dies for all bullet seating. I hold the case flat against the comparator base, and I center the bullet in the comparator before lightly lowering the comparator onto the ogive with consistent light pressure. Nevertheless, I will often get variance from round to round in terms of the OAL to ogive after seating. I attribute this (perhaps incorrectly) to variance in the bullet dimensions. In an effort to deal with the variance, I wind up seating all of my rounds to something greater than what I'm shooting for, and then "walking" down the OAL a thousandth at a time on each round until I get the reading I'm looking for. To say that this is painstaking is an understatement.

    I'd love to just set the seating die, confirm the first round, and then load 30 rounds. However, when I do that, I invaribly wind up with a few that our out of spec by a couple of thousandths -- some lower, some higher. For normal shooting, who cares. When I'm doing OAL testing, however, it matters.

    How do the rest of you tackle this issue? Most importantly, what's causing the variance? Is it bullet dimensions, or is it undulations in the case head causing it to sit less than level on the comparator base? Is it errors in measurement, and am I just not thinking about it correctly? I've fine tuned my measuring technique enough that I'm pretty confident it's not me, but I've been wrong before. Those of you who nitpick like I do, what are your thoughts, and how do you deal with this issue in your ammo?
     
  2. Nimrodmar10

    Nimrodmar10 Well-Known Member

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    You are probably right in thinking it is variation in bullet deminsions. I've seen as much as .007" variation in suposedly match bullets. Try measuring the lenght of the bullet from base to ogive with your bullet comparator and caliper. Then seperate them in groups according to length. Load and shoot the same length bullets as groups of 3 or 5. You'll find that about 50% will be the same length with the others being longer or shorter.
     

  3. Joe King

    Joe King Well-Known Member

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    You are going to find a variance in Ojive dimensions, it's the nature of the beast. what I do is to measure the bearing surface of my bullets, and sort or not depending on my specs for that load.

    As for your seating depth variance you see with a bullet comparator, you need to keep in mid that where your bullet contacts the lands and where your seating plug contacts the bullet, and your bullet comparator contacts are going to be at least 2 different places and likely 3. So you will get varying readings, because it's very difficult for the Ojive to be made the exact same from one bullet to the next. What you can do is to order a seating plug from Redding to fit the bullet your using. then maybe drill/ream out your comparator to contact the Ojive in the same place as your new seater plug.

    Another thing you can do is to shoot Walt Berger an email and ask him this question since BR is his bread and butter. I'm sure he's forgotten more than most of us will ever know collectively.
     
  4. richardca99

    richardca99 Member

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    Hmmm... I didn't know that Redding would make a seating stem to fit a particular bullet. Not sure I'd want to mess with that, as I use MatchKings and VLDs, and I certainly don't want to be changing out seating stems every time.
     
  5. boomtube

    boomtube Well-Known Member

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    I think you're fanning the wind over nothing. Properly developed charges and seating depths are more tolerant of small variations than many think.

    Challenge you to a test; load five each at your desired jump to the lands, load another batch 5 thou longer and another shorter. Shoot the three groups blind, meaning let someone else hand you the ammo so you won't know which batch is which. Then see if there's any significant difference on target.

    I'd bet you can't see any difference unless you're loading on the ragged edge of the good shooting window and, if so, only one of the three groups will be blown but not directly because of the difference but because your seating depth isn't proper to start with.
     
  6. Zep

    Zep Well-Known Member

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    boomtube - you have a very good idea regarding shooting tests blind as I find myself shooting my test loads with a bias.
     
  7. Dr. Vette

    Dr. Vette Well-Known Member

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    I usually load a whole batch long. I then measure each with a comparator, adjust my Redding micrometer and seat them a second time individually so that they all have the same length to the ogive. I find this minimizes variation.

    Thanks to Shawn Carlock for this tip on his DVD:

    Defensive Edge - LRH Reloading DVD
     
  8. Zep

    Zep Well-Known Member

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    Another great idea!
     
  9. richardca99

    richardca99 Member

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    This is what I've been doing. It's slow, but it's the only way I guess to ensure that each round is as precise as possible.
     
  10. richardca99

    richardca99 Member

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    I'll bet you'd be wrong.

    When I do a round robin test of six different seating depths (five shot groups in 0.005" increments), I can most definitely see one to two that are FAR better than the rest, and they are often not consecutive depths. If you'd like to see one such test, I'll happily send a pic. When 0.005" separate the groups, I'd like to not be off by 0.001-0.002" if I can help it. Now, will this prevent me from hitting a steel gong at 600 yards? Of course not.
     
  11. Mikecr

    Mikecr Well-Known Member

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    I agree with Richard. A few of my guns significantly respond to seating changes(moreso than ANY powder amount change).

    I don't agree with the ascertions about bullet dimensions affecting your seating results.
    W/Regard to seating/measurement:
    Base length doesn't matter
    Bearing length doesn't matter
    Nose length doesn't matter
    Ogive radius variances are countered in measured-vs-actual seating depth

    With a longer ogive radius measurement datum moves up the nose -the same amount the seater plug moves up the nose -to cause deeper seating, countering would-be variance in measure.
    Seating can be set EXACTLY with any or all of the above variances.

    Biggest contributors to bad seating: threaded die/press seating(instead of inline/arbor), and seating force variance/excess seating forces.
     
  12. richardca99

    richardca99 Member

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    I'd like to hear your thougths on why the threaded die/press vs. arbor press would contribute to the error...I hadn't thought about that.

    I think I'm starting to get my head around this. Reading an article by German Salazar yesterday helped a bit, though I just had my "ah ha" moment while reading this post.

    I had thought -- though I couldn't really get my head around the "why" -- that if the seating plug contacted every bullet at a fixed radius (regardless of where that radius is along the length of a bullet), then variance should just get pushed into the case during the seating process (or left out, as the case may be). In other words, a good die ought to be able to produce loaded round after loaded round with the plug contact point the same distance from the case head.

    That's all true. The trouble is, the variance that we care about is the variance in the distance from the SEATING PLUG contact point to the COMPARATOR contact point (ogive). Salazar (who ought to know) says that measuring base-to-ogive, base-to-tip, or any other dimension isn't going to tell you squat about this measurement. You'd have to measure seating-plug-contact-point-to-ogive on every bullet, and there isn't a tool to do this. The only way to seat every bullet the same is to seat every bullet individually measure each as you "walk it down."
     
  13. Bart B

    Bart B Well-Known Member

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    If one looks at their rimless bottleneck cases, the point on them that always is at the exact same place in the chamber when fired is the shoulder. It's driven hard into the chamber shoulder by the firing pin and the primer fires very shortly thereafter

    As there's typically a few thousandths spread from shoulder back to case head, bullets seated relative to the case head will have a few thousandths spread in their seater contact point to the case shoulder. Bullet seating reference should be measured from case shoulder to the seater contact point on the bullet.

    The best place on the bullet to contact the bullet seater is the diameter where it first touches the rifling. Other places on the bullet's ogive may vary a thousandth lengthwise from that point as all bullets made with a given set of forming dies will not all have the exact same shape; they vary a tiny bit because the metals in them are not perfectly dimensioned nor perfectly uniform in their metalurgy makup. A couple thousandths spread in the bullet's jump distance to the rifling has no significant effect anyway; that tiny spread in dimensions is not significant to accuracy.
     
  14. Mikecr

    Mikecr Well-Known Member

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    One datum to another on a bullet nose provides for calculation of ogive radius, which affects BC, or what a meplat diameter would end up after trimming, which again affects BC. And yes, it affects pretty much all bullet portion length measurements(invalidating what most think they measure).
    But seating affects of ogive radius variance, as compared in measure, is well under 1thou. That's not to say actual seating depth changes so little, but that counteraction results in so little measured.

    Actual seating depth variances(bearing seated) doesn't matter to accuracy. What matters is distance from ogive contact point to leade angle/distance. I've heard & read many explanations as to why, but nothing explaining why so much, or for some combinations, why so little.
    With my last 3 load workups, the biggest gains were through seating adjustments.
    My seating with Wilson dies measures <1thou variance, and this from Berger bullets mixed in lots(they aren't the same). I haven't seen where the bullet dimensions change seating consistency, provided seating forces are rational and consistent.

    Some people set themselves backward by frequent annealing of necks. While it might(if actually done well) normalize seating forces, it also increases seating forces, which affects seating variances with ogive radius variances.
    But I suspect these people never fully investigated best seating to begin, so they don't see this & they think it's normal to have a lot of variance in OgvOAL.
    I know it don't have to be that way, normal or not.