The Myth about Over All Cartridge Length OACL or COAL

[jcunclejoe]

Over all cartridge length OACL or cartridge over all length COAL is garbage. It is a near meaningless number.

Having that said, please let me explain.

OACL is useful in cases of fitting ammo into a magazine or feeding through some types of actions. That is the only useful application of the number.

The useful number, and the one that should be paid attention to, is the base (of the case) to bore diameter (on the bullet) dimension. This corresponds to the breechface to land dimension in the chamber. The breechface to land dimension must be determined for each and every firearm.

First I will explain why then later how.

WHY
A bullet will contact the lands at the bore diameter. (.300 for a .308 bullet) This bore diameter has only a loose relationship to the tip of the bullet. All bullets vary, from the tip to the bore diameter. They vary from run to run, when die sets are changed, by tooling wear, and the list goes on.
   
A bullet is seated by the seater punch in the die which contacts the bullet at some point (behind the nose or tip) on the ogive. The seater contact point is some distance from the bore diameter. The tip is even farther from the bore diameter. I have personally measured .060 difference between lots of Nosler Ballistic Tip bullets. This is the distance from the seating contact point of the bullet to the bore diameter. From the bore diameter to the tip is even more. I love Ballistic Tip bullets and just use them as an example. They are extremely consistent within a lot. I'm sure others are worse.

So if the seater contact point to bore diameter distance changes with each box of bullets. Then the bore diameter (on the bullet) to the lands (in the barrel) "Jump" distance will change with each box of bullets. UNLESS it is measured and adjusted for each box.

Example:
A bullet is seated to .020 off of the lands. A new box of bullets is opened and seated with the same die with no adjustment from the last loading session. If the seating contact point to bore diameter distance is .060 different? You could be seating those bullets .040 DEEP INTO THE LANDS.
REMEMBER that the bore diameter to tip distance is potentially worse. So if you measure the OACL, it could be .050 or more into the lands.

HOW (to eliminate the variability)

The way to determine the breechface to land dimension is to use a fired, unsized, unprimed, uncharged case and the bullet you will be using.
1. Press the neck of the case against a hard surface, making a small flat on the neck. This will hold the bullet in place.
2. Color the entire bullet with a permanent marker and let it dry.
3. Seat the colored bullet just barely in the case.
4. Chamber the test case/bullet in the firearm. As the bullet touches the lands, it will be pushed back into the case which will scrape the ink from the side of the bullet.
5. Remove the test case. The bullet may remain in the chamber. If it stays in the chamber, a light tap from a cleaning rod inserted from the muzzle end should do.
6. Re-seat the bullet in the case up to the end of the scraping.
This will give the distance from the breechface to the lands when measured from the base of the case to the bore diameter on the bullet.
AT this point you could measure the OACL and subtract .020  This number would be good for this box of bullets. Repeat the above procedure for the next box, etc.

OR

The best way I have found to measure this dimension is with a Sinclair bullet comparator. It is a hex nut with caliber specific holes in each face.

Once this number has been established it should be written down as Maximum (bullet contacting lands) for that particular firearm. All loading should begin at least .020 short of that number, then adjusted as necessary for accuracy, magazine length, minimum neck depth and so on.

The whole idea is to set the jump from the bore diameter of the bullet to the lands in the barrel. Then to maintain it (measure it with the comparator), every time you load.

Unless ammunition is to be carried to the range in a padded case, you should always seat a bullet one diameter into the neck. It helps keep the powder in your case instead of your pocket.

I hope this makes sense to everyone. I hope it is helpful.
Joe

[MickinColo]

Jc,
Nice work. Did you get any sleep last night, this post took some doing.
 

[quickdtoo]

Thanks Joe, nice work....really helps a newbie reloader like me...COL has been a real puzzler for me. If I measure off the ogive with a comparator, the dimension means nothing in relation to the published COL, only a reference for me the next time I load that bullet.

Tim

[Rod B]

Excellent report. I have the Stoney Point OAL length gauge.

I wish I had this info prior to purcasing the gauge. Sounds easy & like all great ideas its simple & effective.

Thanks for sharing your findings.

Rod. :wink:

[longwinters]

I understand what you are saying and agree that technically Ogive is more important than OAL.  But basically anymore all I measure is OAL and it seems to matter little what bullet I am using as my reloads will shoot MOA or better.  I think there is a length range that a bullet will shoot well in a rifle and typically do not think it has to be right on the money to shoot accurately.  This is especially proven true with rifles that have detachable mags which are shorter than the measured OAL of a rifles chamber. I have 2 like this and both are excellent shooters.

Long

[Longcruise]

This is especially proven true with rifles that have detachable mags which are shorter than the measured OAL of a rifles chamber. I have 2 like this and both are excellent shooters

My experience is the same.  My T3 magazine is the limiting factor with most loads but does not seem to be the limiting factor in accuracy with that rifle.  Well built rifles shoot well with a broad range of loads and loading procedures.

I don't have a comparator, but in keeping with my frugal (nay, CHEAP) nature, I've found that by removing the bullet seater from my seating die and using it as a comparator I'm able to match the lengths measured with my COAL gauge ( http://www.mikeswillowlake.com/coal_gauge.htm ) when seating the loads.  If you have ever loaded a batch of pointed soft point ammo of any make you quickly see that measuring off the point will give a pretty broad range of measurements owithin the same batch due to inconsistencys and anomalies in the soft points. (when using a caliper to measure from base to bullet point)

BTW, the coal gauge above is not my creation but I promote it often because of it's simplicity and economy.  My current version does have the case glued into the parent case but instead is a tight push fit so that the rear case can be removed for a more accurate measurement.

[Bullseye]

I cannot agree that it is a meaningless number.  It is a number that will make a load with that particular bullet work in a SAAMI Chamber (and magazine if applicable).  It happens to be the measurement that I reload all my ammo with and have for years.  I continue to do this because I get acceptable accuracy and have not seen great improvements with my style of shooting by seating farther out.

Now I would say we could debate about how closely the bullet is seated to the rifling affects accuracy, but the COL can be used to make loads that go bang with the particular bullets the measurment is for.

[jcunclejoe]

Bullseye, you are absolutely correct, COAL is useful as a place to start and should make useable ammo for any standard SAAMI chamber or magazine.

My intent was to let people know that, only using COAL in the persuit of accuracy would be a mistake. Since accuracy and consistency go hand in hand, the consistency of the jump is far more important than the amount of jump.

I have seen many firearms that shoot very well with a lot of jump, mostly because they are throated like a Weatherby. Also because every individual gun/powder/primer/brass/ bullet combination is unique and some prefer more jump.

I just wanted to let people know that in the persuit of ultimate accuracy, there is a better way than COAL.

Thank you all for your responses. All we can do is learn from each other.

Joe

[Tom W.]

I read something like this years ago, and tried it with my NEF 30/30 using 150 gr. Sierra Gamekings bullets. I ended up With a COL of 2.843, and it does extremely well. I have not done it with my .270 yet, nor with the cast bullet loads for the .30/30, nor with either of my Ruger #1's yet,altho I can foresee it in the near future with the 7mm, mag. The '06 will have to wait a while.

[Whelen Man]

Interesting post.  This has always been a confusing subject.  Another thing to consider when changing the COAL is that when you change the overall length you also change the case capacity.  This brings me to the confusion that is the change in the seating depth causing a change in accuracy due to the relationship of the bullet with the lands or is the change of accuracy due to the change in case capacity.  Long seated bullets give you a larger engine and can be loaded differently.  I'm confusing myself.  :roll:

[Longcruise]

I keep track of COAL in order to be consistent, but the more I play with COAL the less I see it as important in regard to accuracy.  It's very important with some firearms such as the K31 where the throat is almost non-existent.

My now sold NEF 223 could not be seated to come anywhere near the lands because the throat was so long.  It seemed to make no difference in accuracy however and the most accurate was win factory loads with a SAAMI OAL.

My most accurate rifle limits COAL by it's magazine length so that determines OAL.  Accuracy seems to be excellent no matter what is shot through it.

Don't know if that sheds any light or not

[longwinters]

WM, good point.  I hear/read that the fuller you can fill your cartridge case the better off you are.  I tend to believe this, yet when I look in my reloading books it is often the case that the most accurate loads listed are not in the cartridges with the fullest cases. :eek: .  Probably many things in reloading a person can take to the bank on every cartridge/rifle combination.  But I think generalities regarding OAL, OGive and a couple of other things are a little more wide open for variation of thought as proven by experience.

Long

[Slamfire]

My method of determining the head to land distance is a bit different. I insert a flat based bullet into the dummy case nose first. When I extract it, I have the necessary dimension. I'll use a round nosed bullet if the rifle has a long throat in order to get a good grip. I determine the ogive's critial dimension by sticking the nose of whatever bullet I'm going to use into the muzzle, then press and rotate. It may not be good enough for those who have been trained as machinists, but it works for me. Now I can adjust the seater so the mark is of a shorter dimension than the dummy case's length. I can adjust this length to suit my planned distance from the bullet's ogive to the lands of my rifle. When I change bullets I have the head to land dimension available in the dummy round and have only to determine the ogive's contact point.
Starting with the bullet in contact with the lands is not dangerous in and of itself! You just have to work up from a light load until the pressure gets up.  

[lowertroll]

jcunclejoe----If you do as you state in closing and "always seat the bullet one diameter into the case", isn't all the rest of the measuring gyrations a moot point? I don't think I have found a bullet that is close to engaging the lands when seated one caliber.
The assemble, chamber, reassemble method has more variation than that of the bullets themselves.
The manufacturers of bullets set up multiple lines on the same product, so you are getting a mix of parts from those lines in the box you are measuring or loading out of.
I do think that Noslers Balistic Tips are some of the most consistent bullets made in high volume----they must pay close attention to the Juenke machine results during production.

[jcunclejoe]

Lowertroll,
You are correct, the rule of thumb I was referring to is that you should always seat the bullet at least one diameter into the case (For hunting loads especially) target loads can be just about anything that will hold them in the case.

Actually when you re-seat the bullet in the case up to scratches in the ink mark it should only vary by a couple of thousandths, where the bullet curve may vary by sixty thousandths (from the bore diameter to the seating contact point). You do not always have to re-seat the bullet but sometimes it pulls out of the case. Once you try the method a few times it gets to be very easy and you will have good confidence in the results.

You are also correct in that bullets from Speer may be from different production lines in the same box, especially the 22 caliber bullets. Nosler and Barnes run one product at a time per line. If they run two lines of the same product then the bullets are kept separate by lot number.
I cannot speak for Hornady or Sierra but I would expect that they keep lines separated also so that all of the bullets in a box are from the same machine.

Nosler bullets are very consistent in very many respects. The variation comes in the exact shape of the curve of the Ballistic Tip bullets. (Partitions are very consistent over time and from lot to lot) The curve on Ballistic Tips is very consistent within a box but may vary greatly from lot to lot.
It is just a matter of the fine adjustment of the machine that causes the change from lot to lot.

Nosler does not use a Juenke machine. They measure the jacket wall runout (concentricity) with a dial indicator. This is done at least once per lot on several samples. It is better to measure the runout before the bullet is formed than after it is finished as would be done with the Juenke machine.

I hope this clarify's things a bit.
Thanks for the extra questions.
Joe