Bullet Base-to-Ogive Comparator Data

Discussion in 'Gear Talk: What to Buy? and Gear Evaluations' started by dixieppc, Sep 24, 2014.

  1. dixieppc

    dixieppc In search of one small hole...

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    I decided to sort a 100 count box of bullets today. I usually only sort by weight and that has served me pretty well over the past 3 decades. I shoot Berger bullets and they always work out to be real tight on weight distribution. This box of 6mm 105 grain hybrids came out to:
    104.9g 4.0%
    105.0g 77.6&
    105.1g 18.4%
    Since my digital scales are advertised at being accurate to within one tenth of a grain, I weighed each bullet three times to confirmed this distribution. This is the type of distribution I have become accustomed to with Berger bullets and I think this distribution is phenomenal for a mass-produced bullet. Way to go Burger!

    However the weight distribution is not the reason for writing this. I borrowed a friend's Sinclair Base-to-Ogive bullet comparator (bullet sorting stand with dial indicator & 6mm base comparator) and for the first time I sorted a fresh box of bullets based on Base-to-Ogive measurements. The measurement number is meaningless but the percentage of distribution is what we're looking at:
    1.640 14.3%
    1.641 2.0%
    1.6415 2.0%
    1.642 16.3%
    1.6425 4.1%
    1.643 18.4%
    1.6435 2.0%
    1.644 12.3%
    1.6445 10.2%
    1.645 18.4%

    As I said this is the first time I have sorted bullets with a Base-to-Ogive comparator and I'm not sure how to interpret this data as far as what's good and what's not or how far of a spread of piles are OK to lump together. Can somebody educate me on my data.

    Regards.....
     
  2. dmoran

    dmoran Gold $$ Contributor

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    If it was me, I would split them in half. You have a spread of .005" total, so I would put the 1.640 to 1.6425 together as a lot and the 1.6425 to 1.645 to a lot. If you want to hold it tighter yet, split them 3 ways.

    By the way, on say a .500-BC bullet, a 1/2-grain in weight variation will only change the BC less then 0.4% (a miniscule amount). I once tested some 105 A-Max's at 1000yds that had .9-tenths of grain variance in weight. Shot 5 from each end of the variance to a 10-shot group, that all went into a 5.7" nice looking round group. I pay little attention to weight variance, unless it is a large variation.

    My 2-cents
    Donovan
     
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  3. jlow

    jlow

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    That is quite a bit of variation. My guess is there is at least two possible reasons.

    First and foremost, I think (BTW, this is just my own thinking and I would like to think what others thought on this) it is harder to maintain a tight base to ogive length measurement for a VLD type bullet than say an similar weight HPBT bullet.

    The reason is the VLD gets their higher BC by virtue of the fact that their nose is significantly more tapered. When you do a measurement, the comparator is used to measure the ogive but the slimmer the bullet, the more magnified the variance as a slight difference in diameter for a “fat” bullet will only result in a slight difference in this measurement but this is not the case for the same slight difference in diameter for a “slim” bullet.

    The other possibility is the bullets in each lot was not all done by one machine – this is the case with Sierra and not Nosler. In the case of Berger, I don’t know. Only guessing here.
     
  4. dixieppc

    dixieppc In search of one small hole...

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    Thanks for the feedback and insight guys. Let me throw something by you as to the way I'm looking at this and see what you think about it.

    I would think that the closer you are seating your bullets to or into the lands the more the variations in Base-to-Ogive will effect your accuracy. On the other hand, if you have a rifle that likes a bullet jump, the further the jump the less the variations in Base-to-Ogive distance will matter.

    Am I looking at this right or am I way off base?
     
  5. JamesnTN

    JamesnTN Silver $$ Contributor

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    Correct if a rifle likes the bullet on the lands or right at them then yes base to ogive becomes critical. It's only as critical as what your rifle likes as well. Say if it can tolerate .015 in seating depth at the lands saying +.01 to -.005 then you seat your bullets with a .010 jam and sort your bullets so that they only have a .015 variation in base to ogive. Same goes if it only likes .010 or .005 variation
     
  6. mysticplayer

    mysticplayer

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    An interesting test ...

    Take 10 bullets of a known length. Seat them. How does the OAL of the ammo vary now?

    take 10 from another length, seat them. How do these work out?

    how do they compare to the first batch. Repeat as desired with other lengths

    shoot them "blind" - get someone to chamber the ammo for you. Groups using segregated bullet lengths as well as mixed but shooter doesn't know what is being shot.

    How did they group? Was there anything that was a clear winner?

    Jerry
     
  7. BOBC

    BOBC Gold $$ Contributor

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    Uneven amount of lube on the core seated jacket can sometimes cause land contact to base length variation in the final point up operation of bullet making. Also causes diameter variations on the shank of the finished bullet.

    BCB
     
  8. 6BRinNZ

    6BRinNZ

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    Personally I don't see a relationship between base to ogive measurements and TTL measurement with the hybrids because of the boat tail....the variance could be in the boat tail and the ogive profile is the same, or in the bearing surface, with the ogive profile being the same but the bullet has a slightly longer base to tip for example.

    From what I can tell there are two reasons for sorting bullets; 1./ consistency of bullet dimensions for BC consistency. 2./ loaded base to ogive consistency so that precise adjustments can made in seating depth tests.

    A base to ogive measurement with a boat tail doesn't really shed light on either point as it encompases boat tail, bearing surface and ogive profile.

    With testing, I found that very small changes in land to ogive measurements had a large impact on precision when shooting, and importantly I could measure these variances from one loaded round to the next, but variances in BC due to individual dimensions were hard to detect.

    IMO the width of the seating depth node determines what is acceptable in terms of variation not how much it is jumping. For instance it could be jumping .40 but the node only allows .001 variation before detectable group changes are found, where as a jump of .010 has a node that is .005 wide before detectable group changes are found.

    Once I figured out what I could detect on paper (groups) with respect to bullet dimensions I went down a different path.....I definitely don't bother with a base to ogive measurement on a BT bullet any more.

    A lot of guys are using a Bob Green Comparator tool.
     
  9. dixieppc

    dixieppc In search of one small hole...

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    I was thinking of what Jlow said, "it is harder to maintain a tight base to ogive length measurement for a VLD type bullet". So I decided to run another test.

    I took two bullets from my test that were .005 apart on their Base-to-Ogive measurement. I measured them again to confirm that they measured .005 apart and they did. I then seated them into two prepped Lapua 6BR cases. I seat bullets with a Wilson inline seater with the optional VLD seating stem using a K&M Arbor Press.

    I then set the same Sinclair Comparator rig up that I used on the bullets to measure the assembled Case Base-to-Bullet Ogive and instead of getting a .005 difference, I got the same identical measurement.

    With that, I can only agree with what some others have said that Bullet Base-to-Ogive comparisons on VLD bullets are worthless. What matters is Case Base-to-Bullet Ogive comparisons of assembled rounds. If your Case Base-to-Bullet Ogive comparisons are tight then your required seating depth variations will be just as tight dependent on how precisely and uniformly you're able to seat your bullets.

    At this, all you have to worry about is "chasing the Lands". In other words, as your throat erodes, as all throats do, you simply adjust your seating depth to maintain the jump or jam you require.

    Next thing to think about is variations in bullet bearing surface..... Or Not!
     
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  10. snert

    snert Gold $$ Contributor

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    FWIW, and it does not speak directly to this issue, but related...
    '
    I seat long on a turret press, turn the turret to an Instant Indicator, take a reading, turn back to the seat die, dial in the difference on a micrometer die to my desired OAL (to land bearing point) length and seat.

    This makes up for variations in base to land bearing point or "ogive" differences when it comes to making a cartridge /bullet combo that is uniform. However, it does not address the bullet differences like weight alone.

    Maybe sort, then weigh, then follow the above, and then shoot in perfect conditions and...just kidding.

    If one weighed, measured and followed the above, it seems like it would create batches of very similar rounds.

    Snert
     
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  11. jlow

    jlow

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    I think bullet base to ogive is potentially important because it can affect case volume i.e. like when you seat a bullet deeper or shallower.

    Case Base to to ogive is important when you are seating close to or jamming to the lands.
     
  12. JamesnTN

    JamesnTN Silver $$ Contributor

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    If you guys are measuring BT bullets from base to ogive then you're mesuring wrong. To properly measure bullets correctly you need the measure bearing surface. This is where you'll find impact and group variations. To do this with BT bullets you need two bullet comparators setup so you can then measure the bearing surface and sort via this way.
    FB bullets are measured from base to ogive.
     
  13. mikecr

    mikecr

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    Base-to-Ogive is meaningless.
    You're combining 3 separate and different things in this; ogive radius, bearing length, and base length.
    Separate them, and reconsider.
     
  14. dixieppc

    dixieppc In search of one small hole...

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    I had the pleasure of meeting up with some friends this weekend that have been dealing with such subjects as we're covering here for more years than I've been shooting in competitive circles which is now going on 25 years. I would like to share the conclusions of one gentleman whose insight into this makes perfect sense to me.

    First, one of the common denominators between measuring base to ogive and measuring bearing surface is that both measurements are taken by a dial indicator off the base of the bullet. All three gentleman said that they have found a common variation between the measurement of base to ogive and bearing surface measurements. If you get a bullet that has a variation of .005 from the "lot average of bullets with identical diameter" in the measurement of its base to ogive, then more often than not you are going to get that same variation when measuring that same bullet's bearing surface. Therefore it is their assumption that the variations are coming out of the base of the bullets somehow, not the integrity of the ogive or bearing surface.

    One of the gentleman has a machine that I forget the name of but essentially takes the image of an object and blows it up on a grid on a screen where fine measurements can be made that otherwise could not be made by measuring directly from the object itself. Damn, I hope I said that right. Anyway.....

    He took two boat tail match bullets manufactured by a leading bullet manufacturer in the market today that were of the same identical diameter but displayed a .004 variation in base to ogive and .003 variation in bearing surface. He then threw the two bullets up on this screen and measured the flatness of the boat tails and found that one of the boat tails was convexed by .003 more than the other boat tail.

    So he was able to discover the origin of .003 of the .004 total variation in these two bullet's base to ogive measurement and all of the .003 variation of the bearing surface measurement. The remaining .001 variation in the ogive measurement could very well have been a variation in the contour of the ogive.

    So it would appear that the initial .004 variation measuring from base to ogive and bearing surface off of the base gave a false indicator because of variations in the actual flatness of the base part of the boat tail, ie. the Point of Origin for the measurements.

    All precision measurements are based off of some point in the universe that we have confidence in. If that point loses integrity then the integrity of the whole measurement is left in question. In the measurements of ogive and bearing surface, that origin or point is the base of the bullet. In the above example we see that the origin or point of our measurements had a variation of .003 in itself that was projected into the measurements of the ogive and bearing surface measurements causing a false variation measurement of .003 more than the actual .001 variation that was actually there in the ogive and no actual variations in bearing surface.

    So, with the measuring devices that we all have, most of us would have separated these two bullets into two separate piles when in essence these two bullets were within .001 of being identical in ogive and identical in bearing serface.

    Now, I believe that a variation of .003 in the flatness of the base part of a boat tail will have no impact on the performance between the above two bullets. That being said, I believe the above two bullets will be identical in performance barring all other characteristics are identical ie. diameter, weight, concentricity, etc......
     
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  15. mikecr

    mikecr

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    Invalid is invalid, until proven otherwise.
    What if some of the bullets you measure show the same base-to-ogive as lot majority, but they actually have both bearing difference and base differences that counter? Then you would include these bullets with majority even though they hold bearing differences. Right?
    And when ogive radius varies, and base angle varies, you can have all kinds of combinations in measure -that you might think equals bearing variance. Nope..

    Either you isolate each measure for individual consideration, or you're wasting your time.
     
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  16. BenchShooter

    BenchShooter

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    I can see this. Actually, this can work in reverse to. You might have a bullet whose bearing surface is 4 thousands out from another but is showing the same as the other when compared because of differences in the flatness of the base. You would have put both bullets in the same pile instead of separating them and when you shot them you had a flyer and could not figure out why.

    I know that there are differences in the flatness of the base of boat tail bullets but never concerned myself about it until reading this article. I know this because when I weigh my bullets some of them sit perfectly flat in the pan while others rock around a bit. Some more than others. I don't compare according to Ojive or bearing surface so this never really concerned me.

    Because of differences in the flatness of the base of the bullet you could end up separating when you shouldn't or not separating when you should with every bullet.
     
  17. Bob3700

    Bob3700

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    On a boat tail bullet, you measure the actual bearing surface. That is from ogive to the start of the boat tail. The tools don't measure or support the bullet off the base.

    Given the above, what does the flatness of the actual base of the bullet have to do with the bearing surface? What am I missing here?

    By measuring the bearing surface, all I am interested in is the amount of surface area on the bullet that comes into contact with the barrel.

    If you soft seat your bullets into the lands, this should give you the most consistent engagement in the rifling and bullet friction in the bore.

    The above works will enough that single digit ES and SD numbers are the norm in the calibers I shoot.

    All my shooting is prone, sling, iron and scope sights at 600 and 1000 yds.

    Bob
     
  18. dixieppc

    dixieppc In search of one small hole...

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    Quote: "Because of differences in the flatness of the base of the bullet you could end up separating when you shouldn't or not separating when you should with every bullet."

    Exactly, but I didn't finish my story. I'm having to try to write this story in between getting calls and having to run out and I'm losing my train of thought. At the point in the evening when the discussion came to the point that you could not use the base of the bullet as the origin of the measurements, Jim pulled out his old Tubbs bearing surface comparator. The tubs comparator does not use the flat part of the base of the bullet as its origin for the measurement but uses the ogive of the boat tail base as the origin which offers a much more stable origin than the flat part of the bullet base. Problem Solved?(edited)

    However Tubbs no longer offers his bearing surface comparator but the same comparator is now available as the John Buhay comparator (somebody please be so kind as to correct me if I'm wrong). If there is a difference in the two, old vs new, I don't know.

    I'm seriously thinking of investing in the Buhay comparator.

    Regards.....
     
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  19. mikecr

    mikecr

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    More like -assumptions still holding!

    You still have boat tail angle + ogive radius that adjust what you think is bearing.
    I've used a Buhay + Tubb's BSC pieces combined, they are nice, and wrong, until you've qualified the datums.
    I discovered this with a custom taper mic/indicated/software system with which I can qualify the angles.
    It's complicated, and slow going.

    So then with actual bearing differences determined I did some testing and found what I kind of suspected all along; normal bearing variances mean nothing to ES.
    I repeat, waste of time..

    If you care to improve LR bullets, you're better to focus on qualifying ogive radius, and then trimming and pointing off the known good datums at that point. This will atleast improve and normalize BC.
    You can use a Bob Green Comparator(BGC) to qualify ogive radius. And the Hoover pointing/trimming looks best right now.
     
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  20. dixieppc

    dixieppc In search of one small hole...

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    I didn't notice until you quoted me that I made a mistype. I put an explanation point after "problem solved". I meant to put a question mark because I don't know if the problem was solved. As a matter of fact I don't know much about anything when it comes to LR shooting.
    For the past 25 years I have been a 6 PPC benchrest group shooter. The only bullet sorting I have done with 66 grain flat base bullets has been weight sorting. Now that I am journeying into the long range arena I find that I need to start sorting on other variables now. Thanks for your feedback and information. Much appreciated.

    Regards..
     

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