Why list COAL?

[Idaho]

I ask this question because I've had two barrels on my .223 model 10. The factory barrel had a long throat and so a long distance to the lands. A new McGowen barrel that has a throat is 0.120 inches shorter than the factory tube. Measuring OAL to the lands with both barrels and different bullets revealed quite different results. It seems to me that COAL is something unique to each rifle depending on how the chamber and throat are reamed out. My COAL will be meaningless to someone with a different barrel.

The number that will mean more, is how far the bullet is seated from the lands. Bullet jump to the lands greatly affects the accuracy of the bullet; at least from my limited experience.

So, why list COAL? Because it sounds cool? What?

Thanks,

[darkker]

It's about Pressure and burning rate mostly.
OAL tells you bullet depth in the case, and thus case volume. Case volume is the second biggest contributor to a powders shifting burning rate. It also gives a standard reference point against your rifle, to have an idea of how much gas bleed-off and resulting lowered Pressure you will have. To a point (back to case fill) the deeper you seat the bullet, the lower the Pressure because of gas bleed-off.
Without double checking reamer prints, I'm of the opinion that Savage tends to throat their military cartridges with a more NATO throat. Keeps a larger margin of safety for those who love to keep stuffing more powder in cases. Far too many shooters think that unknown/uncalibrated cases and primer cups somehow magically are an accurate reporter of SAAMI pressures. They cannot. So why not toss in a safety step.

I am like you in that Book OAL is never considered, I measure every bullet in every rifle of mine. As long as you don't think you can match/beat book velocities, REGARDLESS of charge weight; you are OK Pressure wise. Just watch velocities as you work-up. Book data is fired in SAAMI minimum spec test equipment, mass produced rifles aren't that tight. So unless you had a barrel built with very specific and tight specs, there are almost never any "fast" barrels, just higher pressures.

[FW Conch]

If I push a bullit into the case to the point of compression, the pressure will "DECREASE"? Are there pressure test charts to illustrate this?

[darkker]

I said to a point, not to compression, can't remember enough of the work to know if he did compression tests as well.
Dr. Lloyd Brownell wrote a book called "Firearms Pressure Factors", in the 60's. Wolff publishing still sells it. Hartmut acknowledges this in QL with his start pressure Kludge. Hornady manuals discuss this, as well.

UncleNick has link to a chart based on Brownell's work, you can see it here:
http://imagizer.imageshack.us/a/img821/2401/seatingdepthvpressure.gif

It is worth mentioning this is only true in rifles. Pistols behave backwards. Seating deeper raises Pressure.

[Idaho]

I appreciate the answer and will admit that I had not thought about seating depth and case capacity although I am aware of it on the few times I've loaded compressed loads. (by the book only) It does appear from the chart that you supplied a link to that seating the bullet out to the lands will create the highest chamber pressure. That was my guess. I've carefully measured the distance to the lands for each of the different bullets that I have. I've settled on 10 thousandths off the lands to load the 75 grain A-Max and am NOT loading to max loads. I really need a chronograph now to see if any change in powder charge brings a corresponding increase in velocity. At some point, hopefully way below max charge, there should be a knee in the speed graph that shows no benefit to extra powder.

Thanks for the response.