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Determining COAL

This article was written in response to a reader asking how to find his COAL without a collamator...

COAL is the cartridge over all length, a vital part of reloading. if the COAL is wrong, pressures can rise to unsafe levels in some instances. Loss of accuracy is another major problem. Determining COAL is about finding the optimum length for hand loads, for your rifle.

Method 1

Magazine empty. Cock the rifle on an empty chamber (do not pull the trigger as the firing pin will protrude and ruin the measurement)
Take a ram rod and carefully insert it down the muzzle to touch the bolt face
Take a plastic clothes peg, slide it down the ram rod till its butted squarely to the bore
Remove the rod a way, then mark where the peg is sitting using a fine tip pen at an angle of 45 deg
Remove the bolt
Dump a projectile in the chamber
Use a dowel, another ram rod or something fine to place against the base of the bullet
Re-insert the ram rod
Re do the peg trick
Mark the rod- same side of the clothes peg as you did last time
Measure the distance between pen marks

This is your max COAL touching the lands. Experiment with 1mm bullet jump to begin with in most instances.

Method 2
Size a fired case so that only 1mm of the lip of the case neck is sized
Insert a projectile so that it is just started into the case neck
Liberally coat the intersection of the ogive and bullet body with lee lube and allow to dry. No lube below the cannelure point / bullet body as the lands will grip the bullet and pull it back out again giving a false reading
Chamber the cartridge, then extract it. This is your Max COAL
Repeat this three times to be sure (if you have a kinetic hammer)

Method 3
Same as above but 5 minute epoxy is used on the inside of the case neck. Once the dummy round is chambered, leave sitting for half an hour or so. I don't use this method but have heard others use it effectively and I can see why.

Method 4
One day when you have a few dollars, buy a comparator and measure COAL off the Ogive: http://www.reloaders.co.nz/Hornady+Bullet+Comparator+And+6+Insert+Set?keyword=comparator&category_id=0

Double checking
Make up a dummy round seated to the final COAL. Lets say the max COAL was 86.5mm touching the lands, your dummy round will have some bullet jump incorporated into it, lets say you want to go minimal at 10 thou / .2mm- so your dummy round is 86.3mm. Take a vivid, mark the Ogive/bullet body junction, let it dry. Dump the dummy round into the chamber, close the bolt. Extract the dummy round carefully, you'll need to guide it out with your fingers to stop the extractor forcing the bullet against the chamber wall, scratching up the bullet. Check for rifling land marks on the projectile, if there are land marks, your initial measurements were wrong. You can now carry on with this and use it as a method for determining the max COAL, setaing the bullet 5 thou deeper before each retest.


Problems:

Bullet sits halfway out of the case neck

Check that your COAL is not so long that the final seating depth is not such, that the bullet sits only halfway down the case neck. If a bullet is only halfway down the case neck, there is a risk that it will not be squarely aligned to the bore. If this occurs, forget about close bullet jumps and seat for concentricity. A good example is the .308 Win and Weatherby cartridges. In the .308 Win, if the 168gr Amax is seated so the the boat tail/body junction is flush with the bottom of the case neck, bullet jump will be around 4mm / 100 thou. The bullet jump may be large but this load will be more accurate than a bullet seated to within a short distance of the lands.

Magazine too short for my optimum COAL

If your COAL is too long for your magazine, such as trying to hand load for a .300 Win Mag in a Tikka T3, you will have to choose between single feeding or seating to the mag length. When seating to the mag length, you will need some room for smooth cartridge feeding from the magazine. As an example, the T3 has an internal mag length of 85.5mm. You will therefore need to seat loads to 84.5mm if you want to ensure smooth feeding from the magazine. In contrast, if developing an accurate load with the A-Max for long range, single feeding cartridges with a COAL of approximately 89mm might prove accurate. Some experimentation may be required, as another example, instead of going forwards and single feeding, you might want to try going deeper, to see how this effects timing (when the bullet leaves the bore / harmonic effects).

Berger VLD

The ogives of VLD projectiles differ from bullet to bullet and can sometimes vary by up to 20 thou / .5mm. To avoid problems, use a bullet jump of 40 thou or 1mm. Occasionally, a VLD will not shoot at 40 thou jump, though I firmly recommend that shooters adopt this bullet jump for preliminary testing. If this jump proves finicky, try a close bullet jump. Oddly, some rifles do like to have the VLD seated close, with some VLD's engaged in the lands while others will be 10-20 thou off the lands. This is due to timing, a sweet spot in the whip of the bore. The barrel in this instance is showing great consistancy. Think of it like a bull whip (if you have ever tried to crack a bull or stock whip). if you don't time your flick correctly, the whip wont crack (break the sound barrier). The user first learns the correct timing, then learns how to do this each and every flick. This is a timed sweet spot producing a consistent result.

Bullet jump

Recommended experimental seating depths (where concentricity and mag length is not a problem) include:

10 thou- metric users round off to .2mm- use especially with SST
20 thou or .5mm
40 thou or 1mm- especially with VLD.
Concentric seating (where the COAL has the bullet sitting out of the case neck)

COAL web

Above, the .308 Winchester cartridge and 168 grain A-Max. The bottom projectile is set to the max COAL touching the rifling.  But if you look at the bullet shank, you will see that there is not enough shank in the case for optimum concentricity. The top projectile is set to the preferred length for concentricity. The optimum length is 71mm (2.795"). (Note: The camera angle is a little misleading here, a slight optical illusion with the reading appearing to be around 70mm).
 

About Free bore


From a machinists perspective (including reamer manufacturers), free bore starts at the case mouth and is measured forwards to the rifling.

From a shooters perspective, free bore refers to bullet jump, measured from the ogive of the projectile forwards.

Same term, two completely different uses of the term.

Going into a bit more depth (for those who want a headache):

Reamer manufacturers use the terms free bore and lead (or leade) interchangeably. In other words, free bore and lead both refer to the area from the case mouth to the beginning of the rifling taper.

Reamer manufacturers use the term throat to describe the ogive shaped part of the reamer. This describes the length of the taper before it comes into contact with the projectile. The throat of a chamber has two important measurements (to the reamer maker), length and angle.

Try not to dwell on this as it can be confusing for industry outsiders. Reamer makers such as PTG are very smart in that if you as a consumer say that you want a long throated cartridge, they translate this into meaning a long free bore cartridge and will work towards this goal accordingly.

A throating reamer cuts both lead and throat angle.

 

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