Measuring base of case to bullet ogive

Discussion in 'Reloading' started by mtang45, Jun 8, 2011.

  1. mtang45

    mtang45 Well-Known Member

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    I am having a hard time grasping why I am getting variation in this measurement. I have the Hornady OAL gage that allows a precise measurement of the distance from base of case (bolt face) to the bullet ogive where it touches the lands. In my mind this should be a "fixed" dimension (bolt face to lands).

    That being said, I consistantly get different measurements for different bullets (brand, type, weight), and I'm confused as hell as to how this occurs. Anyone want to try and make me understand this phenomenon?

    Thanks
     
  2. Kevin Thomas

    Kevin Thomas Well-Known Member

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    mtang45,

    There's a couple of things here that need clarification. If you're comparing different brands or types of bullets, yes, there will definately be variations between them. If you're dealing with different ogives here (which you almost certainly are), then where the seating plug contacts the bullet will affect where it comes to rest with a given die setting.

    It almost sounds as if you're checking factory ammo? Or is this a variety of rounds that you've loaded using different components? If so, the fix is obviuous; you need to establish where the throat is and then seat your bullets using that as a reference point. At least from that point, whatever you load of those components should have the same reading. Switch bullets, and you're back to square one. Also, you need to understand that various bullets will show a preference for more or less jump, and that needs to be experimented with a bit to see what that particular combination favors.

    OAL is a set number, if you're talking about SAAMI max, or a mag length loading. The point at which a bullet contacts the rifling will vary from bullet type to bullet type. The don't necessarily stay the same, either. A 30 cal 168 grain bullet from Hornady will not have the same dimension as a Sierra or a Berger. As I said, the readings need to be taken in context, with the understanding that that dimension is appropriate for that combination only!
     

  3. mtang45

    mtang45 Well-Known Member

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    Kevin,

    Thanks for the input. I'm speaking strictly of reloaded ammo and yes, your example of the 30 cal 168 grain Hornady vs Sierra or Berger is exactly what I am wrestling with. I understand that the ogive will move on the projectile based on each manufacturer's design differences, but why does the measurement from the base of the case to where the projectile ogive touches the lands change from Hornady to Sierra to Berger?

    Perhaps I don't fully understand the ogive; my understanding is that it is the location on the projectile, where the diameter is identical to the inside diameter of the lands; and thus the location of initial contact. It is this location that is measured from the base of the case to determine how far the projectile jumps before contacting the lands. When reloading a 30 cal 168 grain Hornady "jammed" against the lands at xxx dimension, why doesn't this same dimension work for Berger and Sierra? Because really all that we are measuring is the distance from the bolt face to the beginning of the lands and that should be a constant.

    Sorry to be a little thick headed, but I don't understand.
     
  4. Kevin Thomas

    Kevin Thomas Well-Known Member

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    Simple. It's because you're dealing with different ogives, even if they look the same. Even changes within the same brand, caused by switching to a new form die, will show this. As a form die is used, it will occasionally need to be polished. They're solid carbide, but this repeated polishing will eventually wear a "belly" in them and they'll need to be replaced. Over the life of the die, each time it's polished, you may (MAY) see a difference, however slight in where that ogive contacts the rifling. Once that die is replaced, you're dealing with an entirely different die, that is once again on the "small" end of the specs. And that's with dies that are made to be as nearly identical as possible. When you go to a different brand of bullet, you're dealing with an entirely new set of figures. They may look alike, but those two brands will be as different as night and day. They sure won't be interchangable, not if you're looking for ultimate accuracy, anyway.

    The ogive is the radiused portion of the bullet between the meplat and the bearing surface. It's actually a French archetectual term that translates to "pointed arch" like you see in old cathederals. Each manufacturer uses something a bit different, but for the 30 cal 168 grain bullets (let's stick with that theme) they may be around 7 calibers or so. Some may use 7.1, another 7.5, etc.. Differences like this are almost impossible to see, and would need an optical comparator to discern. That, or you may just see the difference in the ogive placement when you measure them on a tool such as your Hornady.

    Let's put it this way; the difference from bolt face to rifling is a constant. When we introduce all the various shapes of different bullets, with the myriad of ogives on the market, THAT dimension (the ogive to the rifling) will change. Any clearer?
     
  5. rscott5028

    rscott5028 Well-Known Member

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    Try an exaggerated picture on paper.

    Draw 2 parallel lines for the bearing surface of your bullet with the distance between them representing the caliber.

    Bullet #1:
    Now, draw 2 straight lines from the end of your bearing surface to a point (meplat) not too far ahead of where the bearing surface stops.

    Bullet #2:
    Draw a second pair of lines from the end of the bearing surface to a point much farther out than the first pair.

    If the grooves are caliber diameter apart and the lands are smaller diameter apart, will Bullet #1 and Bullet #2 touch at the same time? Or, will the 2nd ogive contact the lands first?

    Now, measure the diameter of one of your HNL OAL gage inserts. I just picked up the 25 cal and it measures .245" rather than .257".

    Hope this helps,
    Richard
     
  6. mtang45

    mtang45 Well-Known Member

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    Richard,

    Thanks, you suddenly made the light bulb come on. The insert is not the proper diameter to sit on the ogive. So different bullet designs will have different relationships (as far as distance) from the place where the insert sits on the bullet and the ogive. I had made the erroneous assumption that the insert was the same diameter as, and rested on, the ogive.

    Thanks for helping me out with this gentlemen, it was really bothering me.
     
  7. rscott5028

    rscott5028 Well-Known Member

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    I think you got it.

    But to be clear, the insert does rest on the ogive at some point between the shank and meplat.

    It does so at varying distances depending on the shape of the ogive whether that difference be by bullet design (large variation) or manufacturing tolerances between lots (medium variations) or inidividual bullets (small variations).

    -- richard
     
  8. mtang45

    mtang45 Well-Known Member

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    Richard,

    I understand. I definitely had a misconception of the what the ogive was as well. The ogive is the entire arc from the shank to the meplat. I was defining it as a specific spot, within that arc, where the diameter of the bullet was the same as the lands; which is incorrect and was adding to my confusion. Thanks much for clearing it up.
     
  9. Kevin Thomas

    Kevin Thomas Well-Known Member

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    It sounds like Rich got you straightened out with a good descriptive visual there. Yes, the ogive is the entire arc, from bearing surface to meplat. What it sounds like you've got it confused with is what we'd call a datum line, essentially an arbitrary point at which we're taking the measurment along the ogive, for comparison against other individual rounds. That's where the variations I mentioned earlier come into play, especially if you're trying to compare measurements taken with one tool with those taken using a different tool. Little things that'll add up and sneak up on you if you don't keep an eye on them.

    Here's another visual that might help; take two or three bushings smaller than bullet diameter, say a .300", a .295" and a .290". Insert a .308" diameter bullet, tip down into the bushing. At some point the ogive will contact the bushing and stop it from moving any further. All three will stop on the ogive, but you can see that all three will stop at different points along the ogive. Those would be the datum lines for those respective measurments, and could be compared against other bullets measured in that same bushing. Hope that helps!
     
  10. wyman08

    wyman08 New Member

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    If the insert on the tool used to measure the distance from the bolt face to the lands is undersized resulting in varying lengths from one bullet to the next, how can you adjust your bullet off the lands to minimal clearances? It seems your defeated before you begin if the insert is undersize.
    I've used a re-sized unprimed empty case with a bullet seated well in excess of the AOL. I carefully chamber this round and slowly close the breach seating the bullet against the lands in the process. When extracted and measured it gives me the full distance from the bolt face to the tip of the bullet with the ogive against the lands that particular bullet. I seat the bullets well short of this maximum length and slowly work it out while watching my grouping and indications of excessive pressures.
    As this is actually using the ogive of the bullet I would think it would be more accurate than measurements taken with an undersized insert that would more likely provide measurements in excess of the actual dimension from the bolt face to that portion of the lands that contact the bullets ogive.
     
  11. Mikecr

    Mikecr Well-Known Member

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    wyman08, it's all a relative measure, and the reference really doesn't matter as long as it's consistent to meaning.
    When you soft seat to obtain your max OAL, you can use any tool/datum to define it.
    Log it, and be sure to use the same tool & datum with future measurements(for that chamber).

    There is no benefit in efforts to obtain any 'real' value here, as ultimately you'll be picking a value based on results, and you can do this any way that's consistent & repeatable for you.
     
  12. woods

    woods Well-Known Member

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    All of my comparator inserts ID measurements are .011" less than caliber.

    For instance this .308 caliber insert

    [​IMG]

    Land diameter is typically .008" less than caliber IIRC

    So there is .003" there which will create some variance in distances from case head from bullet type to bullet type, but it should not be much.

    Don't know why they couldn't have made those inserts .008" less than caliber, would have been a truer tool