Bullet bearing surface question

Discussion in 'Reloading' started by jmason, Oct 3, 2008.

  1. jmason

    jmason Well-Known Member

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    I have read most of the threads that cover this topic. So figured I'd give it a try. My problem is that the .277 cal insert for my comparator tool doesn't come up the the crease at the boat tail of the bullets. Is that the way I should go at it or should I use the .277 on the top for ogive and the.284 for the boat tail? The .284 seams to come just a hair past the crease. It is closer to it than the .277. For got to mention that is a 270 win.

    As an aside I did sort them base to ogive already. I have 3 piles in .005 divisions. the bullets (135 smk) had a variance of .015 over 150 of them. Should I sort even further? Like every .002 Base to ogive?
     
  2. royinidaho

    royinidaho Writers Guild

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    Yur darn right. That'll keep ya off the streets awhile. The only person on here who goes to that level of detail, that I know of, is Goodgrouper. However, his rifles do shot very well.

    I've not had much luck with the SMK in the Winny. Nolser Ballistic Tips out done them every time.:confused:
     

  3. Mikecr

    Mikecr Well-Known Member

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    I don't see how typical bearing surface length variance can affect anything at all.
    .005"?
    It doesn't affect BC
    It doesn't affect seating depth(ogive to lands) or release force(from neck)
    It doesn't affect pressure

    What does affect BC, is nose lengths and meplate diameters.
    What affects seating depth, is ogive radius variance.

    There is currently a tool out there to check radius variance. If you seat off the lands, and your gun(your accuracy) is very sensitive to the distance, the check is well worth it.

    It may just be me, but it seems like measuring bearing surface/base to bearing is so common because its easiest of all checks. Just like measuring brass weight is way easier than checking H20 capacity..
    Efforts that might make you feel good(and they can't hurt), but actually fail to connect with direct gains..
     
  4. Eaglet

    Eaglet Well-Known Member

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    Bearing surface does create many different changes that are more visible at the longer ranges.
     
  5. AJ Peacock

    AJ Peacock Well-Known Member

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    Changes in friction will change the pressure curve, velocity and vibration of the barrel.

    Is it enough to notice? At long range, the beating of a butterflies wings can throw the shot off ;)

    AJ
     
  6. Mikecr

    Mikecr Well-Known Member

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    No, No, it's just opposite..
    .005" bearing variance could be significant to a tiny bullet on the edge of any distant performance(like a 20Vartarg at 300yds)(Hell I don't even know if a 20cal bullet has a bearing..), but amounts to absolutely NOTHING in a long range bullet..

    And there are so many variables at 2kyd that it would always be a stretch to make any claims from the results. Think about it, what moa can anyone consistantly produce there? 1moa? 2moa? You think .005" of bearing variance would throw shots(in moa) so far at 100yds, or even 1000yds?
    NO WAY
    Well you could fire up QuickLoad, and JBM calcs, and see for yourself..

    For example;
    Say a 210Berger VLD has a bearing surface of .544
    (.005" is nearly a 1% change)
    If it's caused by reducing the nose length .005 with the same OAL, BC goes from ~.659 to ~.657
    If it's caused by reducing boat tail .005 with same OAL, BC goes from ~.659 to ~.658
    It's a mix, so BC might change anywhere from .001 to .002(or about 1% max)
    At 1kyd the drop difference from 3kfpsMV & .002BC amounts to .3"
    (thats .028moa)
    I am sure your BC varies more than this -anyway-
    I am also sure that any regular 1kyd shooter is working hard to cut 5 to 6" off their groups. So .3" there is probably not big on their 'things to conquer' list.

    I'm not gonna lay out internal ballistics from QL, but a quick check indicates about 2fps increase from a .005 increase in bearing length with the same seating depth. I'd barely see it with 20ft screen spacing on my Oehler. But this would actually make up some of the loss in BC indicated above.
    The barrel time change amounts to 1ns (Nanosecond).
    If your tune requires this, you hurtin..
    And there is always tungsten coating to negate all ballistic traces of this discussion.

    Bearing surface checks can't hurt, unless you're doing it in place of things that really matter.
    But that is what I believe is happening. I think people skip all the difficulties, condensing them down to bullet weight, or OAL, or bearing length.

    Personally, I don't do it even with the best tools. Still working on that 5-6"......
    [that's what she said]
     
    Last edited: Oct 5, 2008
  7. Forester

    Forester Well-Known Member

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    You make a pretty good argument, at least for what level of division would be a good idea. The OP said the extreme spread was .015" though not .005. Following your example that would be something around an inch (if the change is linear and not somehow exponential?) at 1000. I guess everyone can decide for themselves if that matters in their application.

    For myself, I do it because it is easy, does not cost anything, and I am already doing nearly everything else I can reasonably do to improve my reloads so why not?
     
  8. jmason

    jmason Well-Known Member

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    You're right! I don't see too many streets! I am going to try another bullet after I burn these up. I'm looking to do and learn anything that will help. It don't sound like it matters for the distances I'm shooting, but I'll stretch further. My 600 yd groups look like my 300s used to...
     
  9. jmason

    jmason Well-Known Member

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    anyone have an idea if this should work alright?
     
  10. Forester

    Forester Well-Known Member

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    Do you have a tubing mic. you can use to find out what the actual inside diameters are of the comparator inserts?
     
  11. jmason

    jmason Well-Known Member

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    No I don't. I'll end end up getting another .277 insert next time I order some things. I just don't see how I can get an accurate bearing surface measurement with it on the boat tail side since it doesn't actually fit over the crease. For now I know I'm measuring a portion of the bearing surface, maybe not all of it though. I really don't imagine I'm seeing any benefit from it with a factory rifle and shooting the distances a 270 is useful for deer. Maybe I'll learn something in the process. If not it will be something I won't have to learn when I shoot further with or have a custom rifle.
     
  12. Mikecr

    Mikecr Well-Known Member

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    What you're running into is normal and not a bad thing really.
    No doubt the 277 gizzy is drilled to ~.271, which correlates with bore ID rather than groove. A 284 would likely be drilled to ~.277(it's bore ID), which is why it mates very closely to 277 bearing begin.

    It depends on the bullet diameters(which vary with brand and lot to lot) as to how consistantly you can measure. The closer you get to actual bearing, the tougher it is to get consistant measurements because slight changes in contact pressure start affecting the readings.
    You might use 284 gizzys front and back, if your bearings are large enough and you control contact pressure with a bit of precision. This wouldn't be something done for damn with calipers.. You would need a Tubbs/Buhay comparator.