A question that comes up often, and that had me banging my head against a wall for quite some time, tearing down and redoing my dies, is "why am I getting inconsistent C.O.A.L.?"

The long and the short of it is: all projectiles are equal, but some are more equal than others. During the manufacturing process, bullets are formed, but unless you are getting bullets custom-turned on a lathe, the bullets themselves will vary in length. It is all but unavoidable in mass production. It has to do with the tooling used.

NOTE: my set of calipers are what you would find at Autozone for $20, they do not hail from a Swiss watch factory and retail for $800. Additionally, I am not and do not work for a bullet manufacture, this writeup is based solely off of my research on the matter and fiddling around my reloading bench. That being said, ...

At random, I pulled 5 projectiles from a box of Sierra MatchKing .30cal 168grain, and measured them out. These 5 varied from 1.196" to 1.205", a stretch of 0.009(Nine One-thousandths)

Bullet 1


Bullet 2


Bullet 3


Bullet 4


Bullet 5


So why is this even relevant? Well, it really isn't, but yet it is. Basically what IS important is realizing that the bullet is seated by the OGIVE(curved part of the bullet) and not by the tip. This is actually what allows for very consistent seating of the bullet, when you look at it internally of the round.

You can go so far as removing the Seating Stem from your Seating Die to measure a bullet from its Ogive. Borrowing this image from CMP


The bullet is seated by the Ogive because during the production process the tooling used forms the bullet so each will be the same size when measuring from the base to the Ogive, allowing the reloader to keep a consistent Internal Case Volume.


As a matter of fact, those of you who are fine-tuning 300AAC to your gun may be familiar with this concept (this image belongs to Dryflash3)


Here we have all 5 bullets ready to be seated into their respective cases.


Each piece of Hornady .30-06 brass was trimmed to 2.484"


Here are all 5 loaded


These varied from 3.246" to 3.254", or 0.008"

Round 1


Round 2


Round 3


Round 4


Round 5


I understand this is not all-inclusive, and welcome others to contribute. What I am getting at is that if your C.O.A.L. varies, this is normal due to the construction of the bullet. I hope this helps to clean up some of the confusion.

ETA:

I removed the seating stem from my .308 dies(I didn't want to mess with my .30-06 die, it is spot on) to measure the variances of the C.O.A.L. from the Ogive. Ignore the measurement of over 4 inches, the seating stem is long

These measurements range from 4.6215 to 4.6225, or 1/1000th an inch variance. This is the consistency we are looking for. Being as these are cheaper calipers, I am willing to wager that all 5 of these are the same, and that 1/1000ths is my calipers drifting.

Round 1


Round 2


Round 3


Round 4


Round 5


If you are looking to get EACH bullet to be exactly the same, you may want to look into a Meplat Trimmer. I cannot say much about these as I do not have one yet.

Basically what they do is unify each bullet by trimming down the hollow point of the bullet when the machining process tends to be a little off.

This variability is why I load my magazine length rounds a bit shorter than many others.  

You checked four rounds and found 9 thousandths difference.  Imagine what the real variation within the lot is (hint - probably even more).  

I load my 223 rounds to a nominal 2.235" OAL.  I don't want the longest bullets causing a jam.



The other thing I do is make sure the slant height of the loaded round is smaller than the magazine.  For a 223 round, the slant height is about 7 thousandths longer than the OAL.  This ensures that a round that tips in the magazine does not hang up and get jammed.


Enjoy!

What is the variation of those 5 rounds when measured from the ogive?

I'm willing to bet (not very much) the ogive variation is smaller than the COAL variation.

This is what really matters and I have done some base to ogive measuring and found that it can vary as much as .009 as well.

Bullet Prep     http://blog.sinclairintl.com/2010/02/19/bullet-prep/

It's not what matters for rounds which have to fit and feed from a magazine.  For magazine-fed rounds, the rounds have to fit ine the magazine and have a little roo to move around without binding up.  To prevent binding, you need to ensure the longest bullet does not bind.

This is where a Hornady OAL Gauge and Bullet Comparator are invaluable. Only measure from the ogive.

+1

Any update?

No, I'm not impatient.

If I go to the range this weekend there will be. Right now its doubtful. I CAN shoot a rifle on my property, I just don't like to. Damn people and those wooden boxes they live in

Was curious about the variation in regards to measuring these rounds from the ogive.

that, that I can do.

update in the OP

If not already, this should be linked in the "useful threads" tacked thread.  This is great information for new reloaders.  I've seen the COAL question come up many, many times, and this is a great write-up.





Thanks, John87




 

Thanks John, it appears my suspicions were correct.

The variation of COAL when measured from the ogive from quality bullets is much less than the variation of the total round COAL. Very important to know and understand.

Thank you for providing proof for this fact.

I agree with tbonifie, that this should be part of the 'useful threads'.

good photo essay on COAL, sources of variability.  thanks for the good work.

its in the tacked thread as well

For what its worth, when I reload for my precision rifle I sort the projectiles by the length of their bearing surface - the part of the bullet that makes physical contact with the bore.  In a lot 500 175 gr matchkings, the variation between the longest and shortest projectiles was 0.015".  These are 'match' projectiles, so I would guess that in large batches of just about any type of bullet you would see a similar variation percentage.  

I should include the fact that there were only a select few that measured in those extremes.  They usually wind up concentrated around one or two measurements, and you can see the distribution bell curve pretty easily.  But yeah, measuring from the ogive is a much more reliable route, especially if you don't have to worry about magazine dimensions and whatnot.