There's OAL and then there's OAL.

P

Pond James Pond

New member
Surely one of them should remain constant?

I can measure my cartridges based on two points. One is from the base of the case, the head, all the way to the tip of the bullet. The other is from that same base all the way to the ogive of the bullet using a Hornady gauge on my calipers.

I bought one of the Hornady chamber gauge kits and have measured the distance to the lands for every new bullet type I have bought.

Now as I have been playing around with my "Milsurp Maximisation" project I have measured the distance to the lands to set my OAL. This is largely arbitrary as I have a magazine fed gun so that limits my OAL more than the chamber does, but I do it anyway.
I let the bullet sit in the adaptor case, loosen the plunger, push the case all the way to the shoulders, firmly push the plunger forward lock it in place. The bullet may need a tap with a cleaning rod and out it drops. Pop it back in the adaptor and measure with the calipers fitted with the .30 Cal caliper collet. I do it a couple of times more to be sure they are in the same range. Any variation is never huge.

It occurred to me that if I closed the bolt and somehow climbed down the barrel with a tape measure, the distance from the bolt face to those lands would always remain the same.

So it seems to me that regardless of the bullet, that distance should remain the same.
Yet, when I look at my records I see numbers from 56.10mm all the way to 58.30mm! 2.2mm difference.

Since I bought that gauge I have probably shot something like 200-250 bullets max, so it can't be down to throat erosion at this stage.

How is that possible?
Am I wrong about my assumption of a constant distance?
If so, how?
 
My first question would be are you using the same bullets or diff ones? Those numbers can get skewed if you are mixing and matching bullets, as the ogives can vary alot with different type/brand bullets. Every time u change bullet type you will get a diff distance to the lands. You have to remeasure and set seating depth occordingly to keep the same bullet jump. If you ARE using the same bullet and you are sure you are using the gauge right, then those are super el crapo bullets.
Also, do you make these measurements in a clean throat everytime?
 
You're using pulled milsurp bullets for this experiment, correct?

Different manufacturers...different bullets altogether, would certainly do this.

You can have identical weight bullets, with differing profiles (especially secant VLD vs. tangential ogive) that will give you a huge variation in comparator length to the lands.

I had long ago removed "overall length" from my vocabulary, and think only in terms of "comparator length"- except as you say, if mag length is a factor.

Unless in an autoloader, single loading is always an option and we often do it if length requires.
 
OAL, there is OAL and there is OAL, not.

There is MAXIMUM OVERALL LENGTH, is that MOL? Yes when case is omitted. I use COL, I combine over and all to make overall then use MAXIMUM Case OVERALL LENGTH.

I know, I also have a length that is measured from the shoulder/datum to the case head. My cases do not have head space.

F. Guffey
 
Pond, different brands/makes of projectiles have different shapes which makes the way they engage the rifling differ. This is why the measured 'length to ogive' is not the same in your rifle for all projectiles.
 
OAL "IS" OAL

There's always some tolerance in bullet dimensions and often the tip is slightly mis-shaped. The use of OAL is a compromise to accommodate the common measuring equipment used by hand loaders. The bullet shape conforms to a set criteria within accepted standards. Seating dies push against a spot on the ogive while we measure to the tip so there will ALWAYS be some variance somewhere.
I've used foreign made .224 55 grain FMJ that when loaded showed an OAL variance of almost .1" due to the differences in ogive shape and amount of tip damage/manufacturing defect. The results are still "good enough for government work".
 
So it seems to me that regardless of the bullet, that distance should remain the same.
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Nope, you need to re-measure any time you change the batch or brand or type of bullets you use.

Sorry just a fact of life.

Jim
 
The one thing a reloader should not be required to do is start over every day. We discussed the effect pulling bullets can have on the bullet.

F. Guffey
 
It occurred to me that if I closed the bolt and somehow climbed down the barrel with a tape measure, the distance from the bolt face to those lands would always remain the same.
Click to expand...
That distance does remain the same (for X number of shots anyway, until erosion makes it grow)
The differences you see are the inaccuracies in the way you are measuring.

It's impossible to detect minor differences in the amount of pressure you're putting on the case and bullet.

Also as most have said, even if the distance in the chamber itself stays constant, bullet variation will negate any real precision without having some spread in the tolerance

Try using one case and one bullet for several measurements and see if there's not some variation even then.

A friend of mine I used to work with was the QC inspector and he always said on things that were difficult to measure, he'd take 5-10 readings and go with the average
 
The reason you are getting differing lengths (aside from any error in your measuring) is due to #1 where/how you are measuring, and variations in bullets.

The reference point you are using is the point where the bullet engages the rifling, and that point is different on different shape /style bullets.

A good visual example (sorry, I cannot provide pics) is the difference between the "standard" spitzer bullet shape and a "spire point". Both will be the same weight, and diameter, but the difference in shape means a difference in bullet length, AND a difference in where the last full caliber diameter point is on the bullet, relative to the bullet base.

And since THAT point is what contacts the rifling, and is different on different bullets (even with very similar shapes, that is why you are getting different lengths overall.
 
James,

I think the source of confusion is that you are not measuring where the lands make contact with the bullet, but where the comparator insert does, and that's a bit forward of the actual throat contact point. This is because the Hornady comparator is not actual throat diameter. It is a little smaller. So the distance forward of the bullet shoulder (where the ogive begins to depart from full diameter) and where the Hornady insert makes contact with the bullet ogive varies with the radius of the ogive and depends on whether it is tangent or secant geometry. Thus, the tool finds a tool's reference length, but not the actual length to the point where the bullet meets the lands.

You can see the above by coating the bullet with Magic Marker, then using the Hornady gauge to push it into contact with the lands just hard enough that you can make out the rifling marks on the bullet after tapping the bullet out with a cleaning rod. Then remove the bullet from the gauge and measure just the bullet using the comparator insert so you can turn the bullet slightly when the comparator insert contacts it on the ogive. This will leave a ring mark in the Magic Marker ink at the point of contact which will be forward the marks made by the lands. Do this with several your bullets and you will see that distance varies.

The tool finds a position relative to the lands for each bullet for comparison purposes, but it's not an absolute measurement of any kind. It will be different for each shape and it will change as the throat wears. So you pretty much need to make the measurement over for every bullet every time you load a new batch, though if you keep a record for each bullet, then you will find they remain close for awhile. That's actually a good way to monitor throat wear.
 
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Wow. Quite a few responses and quite a few questions. Let's see... first the facts.
The most recent bullets checked were milsurp, and they were the ones furthest from other types but there were still variations between those overall lengths to the lands (OALTL for the sake of simplicity) amongst the Hornady, Lapua and Norma bullets I have also loaded for.

None were bullet with collet pullers: it hasn't arrived yet. These were done with an inertial.

Rifle throat is clean.

However, since typing this response I've seen a further post that I think encapsulates the issue I've been struggling with:

I think the source of confusion is that you are not measuring where the lands make contact with the bullet, but where the comparator insert does, and that's a bit forward of the actual contact point.
Click to expand...

My difficulty was the idea that the distance from bolt face to the lands cannot change, therefore if all my bullets were seated to the lands they too would have the same OALTL and then if I seat all my bullets back 0.02" from the lands then all those distances (OALTL-0.02") should be the same, but all were measured with the Hornady bullet comparator.

So it seems that differences in bullet profile at the point the comparator connects and inconsistencies in my technique could be enough to generate the differences I recorded.

I think I can visualise it now!
 
So it seems that differences in bullet profile at the point the comparator connects and inconsistencies in my technique could be enough to generate the differences I recorded.
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"Technique" can account for a small variation. "Tapping" the rod on the OAL gauge a little harder- for example, and forcing the bullet into, rather than just engaging, the lands.

If you want to see a really big difference in length to ogive, grab an identical weight (say 175 grain) spitzer like the Sierra Match King- and a 175 Berger Match VLD to the point I mentioned earlier.

This is exactly why I can load SMK's in our .260 to .02 off- and still be within mag length, but the Hornady AMax (secant ogive) reguires single loading for the same jump.

You don't even need calipers to visualize this- the differences are so great they're easily seen just by putting the two ogive profiles side by side.
 
This is exactly why I can load SMK's in our .260 to .02 off- and still be within mag length, but the Hornady AMax (secant ogive) reguires single loading for the same jump.
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This is probably my one gripe with the CZ550: I really wish that the Varmint I have would come with the integral magazine of the Standard or Lux: My OALs would not be limited by the box mag.

As you say, some are unaffected by its length whilst others (Amax for me too) are limited. That Amax is my best load so far and I wonder if it would get better if seated closer to the lands or even into them.
 
I think what you are seeing is the different shapes of the various bullets. A thin bullet like the A max will fit differently then a round nose.
 
are you using modified cases (not that it really should make any difference) but I have seen inconsistencies in case sizes which can possibly lead to measurement issues--though obviously different bullet profiles are going to give you different CBTO (cartridge base to ogive). Exceptionally high BC bullets with a long profile can also give "inconsistent" results with a comparator depending on the lead-in to lands, I sometimes have difficulties getting consistent measurements for my 6.5 284
 
are you using modified cases (not that it really should make any difference) but I have seen inconsistencies in case sizes which can possibly lead to measurement issues-
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Getting a bit OT- but I have thought about this, and suspect it might make a difference, but probably not enough to make a significant difference.

If you think about it, the modified cases sold by Hornady are SAAMI standard sized, presumably the same as factory ammo would be. We all know that chambers can vary slightly, even those not intentionally reamed to a different spec.

I plan on making my own by drilling and tapping a case sized to my chamber as I normally use based on the headspace gauge/fireformed brass- and see if there is a difference.
 
are you using modified cases (not that it really should make any difference) but I have seen inconsistencies in case sizes which can possibly lead to measurement issues-
Click to expand...

Difference? It makes a difference to me. I want bullet hold, I want all the bullet hold I can get. Then there is that 'one size fits all', I am the fan of transfers and standards and verifying. I adjust the seating die to the transfer. I do not adjust, remove the case check, place the case back into the shell holder then raise the ran and adjust and repeat the procedure over and over.

I do it one time.

F. Guffey
 
My point is simply that despite the existence of SAAMI specs--you can count on variations in the neighborhood of a couple of thousandths inches in case dimensions, bullets and reamed chamber dimensions. I've encountered instances for example in tight chambers where it can be difficult with an OAL guage to tell when the ogive engages the lands. I have two hornady 300 wm modified cases--one I can close the bolt with no resistance--the other I cannot.

I'm no expert--but I suspect the whole jump to lands thing is a bit over-blown except in the case of match-grade long distance shooting--which is way beyond my skills. I have a consumer grade LRT SASS 308 with an 18" barrel and large jump to lands--yet for some mysterious reason is stunningly accurate--all the more so mysterious because it seems to be quite accurate across wide range of different ammo.
 
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