How to measure COAL with Bullet Comparator

A

akinswi

New member
I know you can get inaccurate readings when using calibers and measure from the tip of a bullet to the bottom of the case. My question is when i measure using a bullet comparator . How do I figure the overall length of the cartridge from the ogive

example i want overall length 3.30
but measure from ogive 2.47

How to adjust my seating die to measure 3.30 based off ogive measurement

thanks
 
Do you want a bullet that is 3.30 so it will fit in a magazine? Just adjust the seating die so the longest round measured length is 3.30 or a bit less. Base to meplat may vary a bit but that is just how it is. Don't over complicate a very basic mechanical operation, simply ignore the base to ogive measurement
 
If you want the OAL to be 3.30", that's the measurement from the tip of the bullet to the base of the case. End of situation. That may be whatever reloading manual you're reading suggests, but that means that round will fit every weapon made. It may not be the most accurate for your gun.

If you want to measure from the base to the ogive, that's different. But you first need to take your rifle (assuming that's the weapon) and drop just the bullet you are using into the chamber. Tap it in lightly with a cleaning rod. That bullet is now right against the rifling in your gun. Put the cleaning rod into the muzzle and slowly drop it to meet the chambered bullet. Make a half-circle with a thin magic marker around the rod. That rod measurement is the ogive at the rifling.

Make a dummy round and seat it long enough that it engages the rifling. Make a full circle around the rod. If there is only one line, your round is right at the rifilng. If the lines don't match, adjust the seating of that bullet until there is only one line.

Measure it with your comparator. That is your OAl-OG = overall length to the ogive for that bullet in your rifle. Now you can seat your bullet back by 0.01, 0.015, 0.02" (deeper into the case) and shoot specimens to see where your best accuracy is.
 
First off, forget getting overall length by measuring to the ogive.
The ogive is NOT a single point. I don't understand why so many people seem to think it is. Go look up the industry standard definition, the ogive of a bullet is the ENTIRE sloped / curved portion of the bullet from the point where it reduces down from full diameter, all the way to the tip of the bullet, which we call the meplat.

what you are doing with the comparator is measuring to a point ON the ogive, the point where it contacts the rifling leade. And that point will be different with every different design of bullet and every different rifle barrel.

If you want to measure from the base to the ogive, that's different. But you first need to take your rifle (assuming that's the weapon) and drop just the bullet you are using into the chamber. Tap it in lightly with a cleaning rod. That bullet is now right against the rifling in your gun. Put the cleaning rod into the muzzle and slowly drop it to meet the chambered bullet. Make a half-circle with a thin magic marker around the rod. That rod measurement is the ogive at the rifling.
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I disagree. As described, your method simply will not work. It will not give you the point where the bullet ogive touches the rifling. It can't.

What it gives you is the point where the cleaning rod touches the bullet, nothing else. And the point where a cleaning rod stops against the bullet is well short of the point where the bullet touches the rifling.

DO you have a "cleaning rod" that can reach all the way down to where the ogive hits the rifling? I don't. An ordinary cleaning rod, without a tip loop/jag will have a threaded hole for that tip to screw into. SOME of the bullet tip MAY fit in that hole, with certain bullets in some calibers (more for smaller ones) but no rod has a hole large enough to accept the bullet far enough to reach down to where the bullet ogive touches the rifling.

With your method, as described, what you will get is a measurement from where the rod stops on the bullet, back to the muzzle. And, with some bullets and calibers, where the rod stops against the bullet could be the tip of the bullet itself.

Cartridge over all length is measured from the base of the case to the tip of the bullet. Measuring from the case base to a point on the ogive doesn't tell you squat about the overall loaded length of the round.

Telling people it does is giving bad information.
 
Akinswi,

The method is simple, but it will be different for every bullet design and it will be specific to your comparator insert.

First, use your caliper to measure the length of a number of your bullets so you can determine the average length (with hollow-point match bullets, it won't be identical from one to the next). Second, set up the comparator on the caliper. Close the jaws and zero the caliper. Measure the bullets you measured before using the comparator insert. Average the result (it will be shorter than the bullet length). Subtract the average bullet base-to-ogive measurement from the average bullet length. The result is the average bullet tip-to-ogive measurement for that particular bullet type when using your particular comparator insert. Subtract that number from your desired COL and you have the number the comparator will give you on a loaded cartridge when the cartridge is that desired COL (assuming you always remember to zero the caliper with the adapter in place before you start measuring cartridges).
 
44AMP, your consistent argument is not refutable, and I have no intention of doing so. But the common reloader has come to consider the point at which any bullet engages the rifling is at the ogive of that particular bullet. A .270, 150gr bullet from 5 manufacturers may have a different point at which the surface of the bullet contacts the rifling. That point is conveniently called the ogive. If you call the ogive an unidentfiable part of the curve of the bullet, that serves no useful purpose. NOTE what Unclenick said in his post above:
"Close the jaws and zero the caliper. Put the average bullet into the jaws so the caliper measures from its base to the place the insert touches on the ogive."

With respect, I refer you to John Wooters text, The Complete Book of Practical Handloading, copyright 1976, pages 114-115, wherein he describes exactly what I posted, and where I obtained that information in 1976, and have used that technique to establish the distance of my bullets from the rifling ever since.

The cleaning rod does not have to end ON the "ogive" ( which most of us consider that point that stops at the rifling). The bullet is unable to go any further in the barrel because it is stopped by what is conveniently called, the "ogive." The measurement from the muzzle to the part of the tip of the bullet that touches the cleaning rod is just a marker. When you make a dummy round, if the cleaning rod touches the bullet at the same distance you have marked on the rod, then that dummy round is also at the rifling. You then measure the cartridge with a comparator and seat the bullet 0.01, 0.015, 0.02" or whatever deeper distance you wish to move the bullet contact point (aka ogive) away from the rifling.

How do you establish the desired seating depth?
 
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44AMP, I looked for "What is the ogive of a bullet?" on the net. These are the first 4 items presented:

https://www.larrywillis.com/bullet-shape.html
The curved shape of a bullet (called the ogive) is more consistent than the actual tip on most bullets. Therefore, you can verify the Overall length (OAL) of your handloads more accurately by measuring back from the ogive to the base. Your chamber pressure and accuracy is affected a great deal more by the positioning of the bullet ogive than by the actual bullet tip

This agrees with what you have said:
What is an ogive bullet? - Quora
https://www.quora.com/What-is-an-ogive-bullet
When applied to ballistics (or aerodynamics) an ogive is a pointed/curved surface mainly used to form the approximately streamlined nose of a bullet or other projectile, reducing air resistance or the drag of air.


What does Cartridge Base to Ogive (CBTO) mean? | Berger ...
https://bergerbullets.com/.../what-does-cartridge-base-to-ogive-cbto-mean
Cartridge Base to Ogive (CBTO) is the measurement from the base of a loaded round to a specific point on the ogive (nose) of a bullet.

1. How to Measure Bullet Seating Depth – Panhandle Precision
panhandleprecision.com/measure-bullet-seating-depth
Mar 05, 2017 · The two measurements discussed in the video are Cartridge Overall Length (COAL) and Cartridge Base to Ogive (CBTO). Both need to be considered when seating bullets. COAL is measured from case head to bullet tip. CBTO is the measurement from the case head to where the bullet contacts the barrel’s rifling.
 
simple answer


1. set the die so the round has a COAL measurement of 3.30 inches.
2. Measure BTO with your comparator.

you now know what the base to ogive measurement need to be to give you a COAL of 3.30 inches.

;)
 
This is one of those cases where a misuse of terminology takes the conversation off into the weeds.

"I need for you to understand what I said was not what I meant."

Cartridge Over All Length is simply what it says it is. Its generally important to fit magazines. IIRC its 2.260 for 5.56 and 2.800 for 7.62 NATO.
That is the functionality of the COAL spec. When you say "COAL", thats what the conversation is about.

I take it that you are interested in a different functional relationship. Thats OK,but I suggest you don't confuse the issue by calling it COAL . Its not.

Different manufactures and even different catalogue numbers will have different ogives. An unofficial top of my head definition of "ogive" is the contour of the bullet from the cylindrical diameter to the meplat. "The Ogive" is the entire forward part of the bullet. It is NOT the precise point of contact with the lands. We might talk about shapes such as "secant ogive" or "tangent ogive" The "ogive" of a 175 gr Sierra Matchking is defined when it is in the box. It does not change depending on which barrel it goes into.

We can come up with a useful comparative reference dimension by resting a circular "datum" on the ogive.


I take it you are actually concerned with how far the bullet is "off the lands"

That can play a part in fine tuning for accuracy. The language for that is "I'm loading them .015 off the lands."

A difficulty you may run into,thats literally a moving target. The lands erode.

Via whatever method you choose to use to establish at what seating depth your ogive contact the lands,thats your reference...on that day,with that lot of bullets.

You can use your comparator ,which applies a circular datum to the ogive,to gather a reference dimension. There is no reason to believe that comparator datum circle represents your actual barrel contact diameter. Barrels vary.

Its simply a reference landmark for you to record as a benchmark or baseline. By itself,it has no meaning. Its a point from which you measure change.

It might mean the change of ".020 off the lands" or it might mean "Hmmm. After 350 rounds,This exact seating depth now places my bullet .027 off the lands and my group opened up. MAYBE I should try a .007 less seating to compensate for throat wear"

My point is accuracy of terminology.
 
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Reloading manuals specify cartridge overall length because in many cases (e.g. the majority of self-loading pistols and rifles), this is the absolute limit on how far the bullet can stick out of the case (to fit into the magazine) or into it (to avoid charge compression and resultant overpressures if close to maximum). AR platforms don't care that your bullet wants to be closer to the lands. There's only so much room. Bolt actions are more forgiving (unless the bolt stroke or the cut for loading and ejection is so custom-fit small that it limits COAL).

Measurement of base to ogive is exquisitely dependent on the diameter of the ogive end of the measuring tool and the actual shape of the bullet. A secant-ogive bullet might have that measurement in a very different place from a round-nose, and a wadcutter doesn't respect it at all.

I understand that what the BTO crowd is trying to do is determine the absolute longest the throat of their rifle will allow them to seat out (longer is unsafe) and then tune for accuracy from there, but you have to understand exactly WHAT you are measuring, WHY you are measuring it, and where the potential uncertainties arise.

The most important thing to know with these comparators is exactly where they are measuring on the bullet (or the case, for shoulder-length comparators). Recall that they locate a point at which the bullet or the case has a particular circular diameter, and you need to know what that is and understand why you are measuring to that point.

They offer a RELATIVE measurement. Change the shoulder of the case (e.g. by fireforming), change the shape of the bullet you are using, and the measurement becomes meaningless; you have to establish new reference standards for your own use.
 
Technically the term CBTO should mean from the cartridge base to the point where the bearing surface ends and the Ogive begins. A more correct way of stating it would be cartridge base to bore datum, meaning the diameter point on the ogive that coincides with the bore diameter. But as with most things in language evolution the term CBTO has come to be understood incorrectly as used by people in general and so it’s actual meaning isn’t the same as it’s understood meaning.
 
SAAMI's glossary says the ogive is the curved portion of a bullet forward of the bearing surface.

No mention of a specific place on that curved area.

Depending on the bore and groove diameters as well as the leade angle, the bullet ogive diameter that first touches the rifling will vary.
 
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Here's an idea that will blow your mind. Load up the cartridge and see if it fit's. If it's to long seat it a little deeper till it fits. I do not see where knowinf that number makes any difference at all!
 
specific place on that curved area
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that would be the datum and the datum for where the ogive first meets the rifling and the datum where the comparator meets the ogive and the datum where the seating stem meets the ogive are probably three different datums :confused::confused::confused::confused:

datum in American English

1. something known or assumed; information from which conclusions can be inferred. see also data. 2. a real or assumed thing, used as a basis for calculations or measurements, as a level (also datum plane) from which elevations and depths are measured in surveying.



https://www.collinsdictionary.com/us/dictionary/english/datum

did I sufficiently over complicate a simple operation and confuse the situation ? :D
 
"Technically the term CBTO should mean from the cartridge base to the point where the bearing surface ends and the Ogive begins."

My opinion is, that is what everyone who uses the term, "ogive," means to the user.

Is there any value in handloading to know or use the true definition of "the ogive of a bullet is the ENTIRE sloped / curved portion of the bullet from the point where it reduces down from full diameter, all the way to the tip of the bullet, which we call the meplat".
 
that would be the datum and the datum for where the ogive first meets the rifling and the datum where the comparator meets the ogive and the datum where the seating stem meets the ogive are probably three different datums

datum in American English

1. something known or assumed; information from which conclusions can be inferred. see also data. 2. a real or assumed thing, used as a basis for calculations or measurements, as a level (also datum plane) from which elevations and depths are measured in surveying.



https://www.collinsdictionary.com/us.../english/datum

did I sufficiently over complicate a simple operation and confuse the situation ?
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Makes sense to me--might as well further complicate things and bring in headspace as well. :D That's when the logic of Cortina's "jam and back off" starts to make more sense.
 
I have a bolt action that I wanted to use for accuracy at long distance. I want every cartridge to fit in the magazine but I do not want to single feed. Here is how I did it.

I pulled a number of projectiles from my pile, enough to be a representative sample. I measured bullet base to that point on the comparator where contact is made, then measured the same bullets from the base to the tip, then subtracted the first measurement from the second. That gives me contact point to tip. Then I calculated the standard deviation and multiplied by three. Magazine length minus three standard deviations is now my target COAL.

Then I did seating depth testing to determine the best seating depth for accuracy starting with maximum COAL and deeper seating (longer jump) but with 100% reliability of magazine fit. I don't really care what accuracy I might be able to get by single feeding.

Once I find that, my quality check after seating is measuring from cartridge base to contact point. I don't bother measuring COAL. I know that all will fit.
 
I have a bolt action that I wanted to use for accuracy at long distance. I want every cartridge to fit in the magazine but I do not want to single feed. Here is how I did it.

I pulled a number of projectiles from my pile, enough to be a representative sample. I measured bullet base to that point on the comparator where contact is made, then measured the same bullets from the base to the tip, then subtracted the first measurement from the second. That gives me contact point to tip. Then I calculated the standard deviation and multiplied by three. Magazine length minus three standard deviations is now my target COAL.

Then I did seating depth testing to determine the best seating depth for accuracy starting with maximum COAL and deeper seating (longer jump) but with 100% reliability of magazine fit. I don't really care what accuracy I might be able to get by single feeding.

Once I find that, my quality check after seating is measuring from cartridge base to contact point. I don't bother measuring COAL. I know that all will fit.
Click to expand...

that would work

I am in the process of building a .308 repeater and will be seating all bullets to 2.75, SAAMI specs minus .005 to allow for ogive/meplat differences. Rather than tune the seating depth I will be installing a tuner once I have the best grouping from adjusting powder charge
 
If all the ammo's so dimensioned, the bullets will have an additional .001" jump to the rifling every 30 shots.
 
Not at all. The BTO measurement is determined by two points. The top of the stroke on the ram and the point where the stem of the seating die contacts the bullet. Once the die is adjusted and if you use a full stroke on the press every bullet should come out the same.

I loaded 80 rounds for a upcoming match. Randomly checked 20 for concentricity and BTO. No fancy equipment, just a off the shelf Forster seating die in a Lee Turret. All BTO's were exactly the same according to my Starrett calipers
 
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