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Measuring base of case to bullet ogive

 
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  #1  
Old 06-08-2011, 11:19 AM
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Measuring base of case to bullet ogive

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
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  #2  
Old 06-08-2011, 01:12 PM
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Re: Measuring base of case to bullet ogive

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!
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Old 06-08-2011, 01:30 PM
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Re: Measuring base of case to bullet ogive

Quote:
Originally Posted by Kevin Thomas View Post
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!
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.
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Old 06-08-2011, 01:46 PM
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Re: Measuring base of case to bullet ogive

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?
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  #5  
Old 06-08-2011, 02:18 PM
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Re: Measuring base of case to bullet ogive

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
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Old 06-08-2011, 03:04 PM
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Re: Measuring base of case to bullet ogive

Quote:
Originally Posted by rscott5028 View Post
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
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.
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  #7  
Old 06-08-2011, 03:21 PM
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Re: Measuring base of case to bullet ogive

Quote:
Originally Posted by mtang45 View Post
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.
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
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