Cartridge Over All Length (COAL)
Hi folks Eric here again
This is the 3rd topic in my reloading conversation
Cartidge Over All Length or COAL (This is more complex than my following narrative suggests – do your research!)
COAL is simply the distance from the base of the cartridge to the tip of the bullet and is defined in the SAAMI specs and reloading manuals as an absolute length that is a manufactures standard. This published dimension has no relation to a particular rifle but is meant to define loaded ammunition dimensions that will fit all magazines or chambers that follow the standard.
Hand loaders seeking best accuracy know that the COAL published numbers mean little to accuracy and are not absolutes. For Black guns COAL is often the magazine length but for bolt guns (what I am talking about) means the distance from the bolt face to the end of the throat where the rifling engages the ogive (pronounced ojive) of the loaded bullet. When I refer to COAL I mean the distance from the base of the cartridge to the bullet Ogive as measured by comparitor. In other words COAL is the length you record in your journal or log of the cartridge that lets you recreate the rounds you develop.
This is the second part of the accuracy puzzle. After matching the cartridge case to rifle chamber, the relation of the bullet to the lands and groves in the barrel is usually the next most important aspect of accurate and consistent ammunition. This statement is not exactly true since the impulse of the powder, primer and bullet combination play a deciding role in accuracy, but from a reloading standpoint, COAL for a given appropriate projectile and powder charge is the next step in our search for accurate ammunition.
For accurate ammunition we usually seat the bullet consistently short of the lands and grooves in your barrel. The transition between the lands and the chamber (called the throat) varies between individual rifles and so you need to measure this distance. There are many ways to measure this including the Stony point tool and Frankford arsenal's tool, but my favorite way is to modify a new case to lightly hold a bullet seated long, chamber it and measure the length of the resulting cartridge.
To do this I take an undersized piece of brass (never fired brass works well) and slit the neck using a dremel tool in 2 places down to the shoulder. sand or polish the rough parts on the case and long-seat a bullet you want to develop a load for. Smear sizing wax on bullet and carefully chamber the unprimed round. Carefully extract the round (it helps to remove the firing pin assembly from your bolt before) and measure with your comparitor the length of the round. I use the digital device from Innovative technologies which works best for me. This measurement is the distance from the bolt face to the start of the lands and is described as the “Jam” length and represents our “max” COAL.
Now that you have this measurement for this particular bullet (You need to do this for each different brand and style of bullet) we can proceed to load development. I usually research information on the internet for my particular cartridge and bullet and seat the bullets 20 thousandths of inch short of the “jam” COAL for load development. To clarify, if we imagine a number line like in math class then “jam” =0 and -20 = 20 thousandth inch shorter than touching lands, +20 jam means we seat bullets longer than jam and when we close the bolt the bullet is pushed into the case 20 thousandths. + Jam loadings will raise the pressure of a given cartridge loading and must be approached with caution.
So – COAL in our accurate reloading world means different things for different rifles and bullets. Do you have questions? I find that if you have a question it is easiest to get a solution if you can ask or discuss the question, so fee free to ask me or look on forums to get a good answer.
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