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Measuring base of case to bullet ogive

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Unread 06-08-2011, 03:54 PM
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Join Date: Apr 2005
Location: Missouri
Posts: 241
Re: Measuring base of case to bullet ogive

Originally Posted by rscott5028 View Post
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

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.
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Unread 06-09-2011, 09:59 AM
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Re: Measuring base of case to bullet ogive

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!
Kevin Thomas
Lapua USA
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Unread 06-14-2013, 04:09 PM
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Join Date: Jun 2013
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Re: Measuring base of case to bullet ogive

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.
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Unread 06-14-2013, 05:54 PM
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Re: Measuring base of case to bullet ogive

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.
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Unread 06-15-2013, 09:51 AM
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Re: Measuring base of case to bullet ogive

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

For instance this .308 caliber insert

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
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