OAL Measurements why are they different?

Ok experts I have been wondering for quite a while, Why do you get different measurements from the case base to the Ogive on different bullets.
The only one that I can come up with is that I don’t have a tool that precisely matches the dimensions of my chamber esp the lands dimension.

To me I would have thought that if you did have a tool that matched your chamber then the length from the case to the end of your measurement tool would always be the exact same length. If the different measurements you were making were always of a case with a bullet touching the lands with the same pressure ie not jammed, not off the lands. As the distance from your bolt face to your lands never changes.

Actually, I think that your tools are probably the most constant variables in this.

Bullets are just a tiny bit different… The bearing surface on the projectile can be a little longer or a little shorter, so that would give a little variance when measuring from the ogive. You may have a fraction of a mm differences in your seating depth. For example the press or the bench flexes a little bit on some strokes but not others. Or you’re applying different amounts of force on the downwards stroke or your shell holder has a little play in it. It could be that base of the case is of variable thickness. Add all of that together and there’s your variance, said @juststarting while educated guessing :slight_smile:

Out of interest, what’s the extreme spread on that?

I have a couple of tools I use for the purpose one is a home made jobbie that I tried to replicate the bore of my rifles. It sits very deep over a projectile the others I use are Redding Seating stems that I use separate to any dies. They give different results depending on the projectile shape. Where the Ogive is on a projectile is another point. I assume it is at the point that is meant to make contact with the lands on most guns.

I have it fixed in my head that is should be the same if only the comparison tool was exactly the same diameter as the barrel.

It is not so much the seating depths once up and going that I am concerned about. I have observed plenty of variance in the projectiles also the seating stems in the dies even the very expensive ones do not push or contact on what I believe is the Ogive so that make a difference and why your dies need adjustment for different bullets. As you say then there is the mechanical tolerances of seating esp if you have any sort of compressed load.
All that is why I leave at least 5 thou tolerance on any measurement if I want 10 thou jammed I will adjust for what I think is 15 that is the +/- 5 like wise for 20 thou off I normally set for 25. I have not found a projectile to be that sensitive yet that this effects the group.
The stage that I am really interested in is that first measurement when I remove the ejector from the bolt and do the testing to find the depth of that projectile and the measurements that I take. It is here that I think if only I had that special comparitor every measurement should be the same.

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5,000 of a millimetre is a pretty right tolerance, mate.

Its old school measurements 5 thousands of an inch still small at 0.125mm. It is a small measurement and the calipiers only have a resolution of 1/2 of 1 thousandths. Getting the projectile all straight on all sides can be a big source of errors. Then there is when you call it “on the lands” I think mail call monday on 511 has a good video. Its not when you can close the bolt with some small resistance it is just at the point that the bolt flops closed by it self. Also a good indication it when opening the bolt. There should be no click and slight spring backwards on the bolt. I think I have refined this measurement about six times becoming more critical each time.

I’d just put it down to bullet manufacture tolerances in the radius of the ogive giving a slightly different contact point, both on your seating stem and comparitor.

I know… Just pulling your leg. Because millimetres :slight_smile:

Even “match” grade bullets are not immune to variation. I have weighed match components and have found variations of up to 4gr in 168gr projectiles. If that is allowable tolerances in weight, ( 2.3% ), what is the allowance in shape?
As an aside, I cast bullets for my bpcr rifles to a tolerance of 0.3gr in a 550gr bullet.

Bugger I am so easily suckered

@juststarting, while I agree with you about the comment to the effect that, “There’s a tiny bit of variance” from bullet to bullet, that can be resolved by simply taking measurements of weight (to the 1/10th of a grain of course) diameter and lengths, (to the 1/10ths of 1/1000ths as in 0.0001" inches) and if y’all use the metric system, you’ll just have to convert everything… Oh and back to my point, the bullet and brass makers have tolerances that they work within. Once y’all find those tolerances by your careful measurements, all you have to do is sort and group the bullets that are similar in weights and sizes…

Next question is how closely do I group them…? Answer: within 0.0002 of an inch, and within 0.2 grains of weight. running more than that begins to start showing itself in groups…


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I sort my bullets by weight down to batches of 0.015 gr. The cases are sorted to 0.15 gr groups. This sorts out a lot of other dimensional variances just because.

Most bullet manufactures produce and sell product that is outside their advertised specs. Sometimes they seem to go through phases were they are really on the ball and everything is great. Then they drop the ball.

I’ll have a crack at this cause it’s something I’ve spent considerable time getting my head around. The ogive of a bullet is where it first engages the rifling as we know but why does it vary so much round to round and even bullet to bullet? Unless the gauge & die contact the bullet at exactly the same point when used then you’ll have this problem due to the variations in the bullets profile. In order to eliminate this you need to cast the chamber, measure the lands and precision machine a collar for the comparator. The next step is to uniform the Meplat by either trimming or pointing then trimming. Once uniformed you will have a consistent Meplat to ogive and ready to then sort by bearing surface in .001" increments.
I use the Hoover Meplat Trimmer with a custom body made for the 350gr SMK which I buy in packs of 500. It’ll take me the best part of 15-20hrs over the course of a week to point, trim, measure and sort into lots ready for loading. When It comes to seating, I will seat long then measure with the gauge, subtract the difference and adjust the micrometer on the die for the final seating depth. It stands to reason that the brass has been prepared to a similar level of anal retentiveness.
I only do this for one rifle (375 Cheytac) because it’s a very time consuming tedious process and this is the only caliber I kill things with at distance. The other big guns all use precision made CNC projectiles so there’s no gains to be had measuring them.

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@Brett, have you taken a look the Forster datum dial kit, it has those collars you’re talking about, sir… But I also don’t know if such a tool is exported to your country? I might try to find out, unless you would rather I not bother…?

Thanks @Brett so I read your post and then was going to do a reply but then read it another couple of times as it sounds you have and a think about this one too.
Would you say if you had the cast of your chamber and that magic collar that all other things being equal “in tolerance” that you should get the same measurement with any bullet no matter shape or manufacturer?
It sound like you are putting it the differences down to measurement equipment “collar” and then bullet to bullet differences which would be the much smaller component given the size of the projectiles I am working with and them supposed to being a quality product.
In fact I have measured a lot of them placing them into my collar and measuring my Ogive datum point to base and they are pretty consistent.

Ideally all being equal then yes you should get the same measurement but only if the seating die insert contacts the ogive at the exact same point. I believe there’s one of the die makers that will machine up a custom insert but can’t recall who it was. As you probably have worked out, each different make / type of projectile will have the ogive at a different point along the length which is why I record the max OAL for each projectile used for other rifles besides the Cheytac. Casting chambers is a painful process and I only did it with the Cheytac because the barrel is easily removed.

@Jim_Fleming I have the Forster kit along with the Hornady LNL case gauge and while close they are not precise enough for my liking. I have a pile of bar stock, a mini lathe and precision adjustable reamers to make my own inserts. I’ve only made a couple as my time is limited and I only shoot out past 1000yds with 2 calibers, 375 Cheytac and 50BMG. Every other rifle has a case that I’ve tapped for use with the Hornady gauge and I record the max OAL for each projectile type in my load data. Where in the US are you? I’ve got family in Santa Barbara and I’ve competed at NRA Whittington just outside of Raton NM.

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Just to be pedantic: the ogive is actually the curved section of the bullet between the meplat (point/tip) of the bullet and the straight sides of the bullet or bearing surface. The place where the rifling lands contact the bullet is the start of the ogive or the shoulder of the bullet.
The term ‘ogive’ is derived from a pointed arch in gothic architecture:


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Cool pic :slight_smile: Just keep in mind that the rifling has a slightly smaller diameter then the shoulder depending on the depth of the rifling. Throat angle also comes into play as to where the bullet first contacts the rifling. Then we can throw that out the window if using Bore Rider bullets :slight_smile:

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Yep. Good point. First contact is probably just forward of the shoulder, generally speaking.

@Brett, color me jealous! I’ve been to The Whittington once, and spent two solid days of having a blast on the Highpower Silhouette Range.
We live in North Carolina, Brett. Having recently moved down from Pennsylvania.
Thank you for asking. I live very far away from the left coast of this country and those people in California.

Haven’t ventured that far east in the US yet but Hickok45 is next door in Tennessee and I’d like to visit him someday so maybe I’ll get there. I should be back in the US this time next year if I can get some sense from the government about what permits I need. I got flagged by Homeland Security late last year when trying to export some brass from the US and that may have caused issues this time. Wasn’t anything dramatic but someone got their panties in a bunch about empty 20mm Vulcan cases…Was bad enough that the Australian Federal Police reamed me over some scope mounts and wanted a declaration that I didn’t intend to fit them to military rifles and wage war on Australia or our allies, I kid you not.

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