The difference in OAL is due to the different lengths of bullets of the same weight. These lengths vary because of the shape.
A 124 grain FMJ may be shorter than a 124 grain JHP (the "hollow" in the JHP means that the bullet will have to be longer to contain the same 124 grains of material). But there are so many different JHPs and FMJs from so many companies, and there are so many variances in shapes of those, that you really can't be certain how any one will compare to any other.
It's a bit confusing because we use overall length as our measurement, but we really don't care about the overall length of the round. What we really care about is the amount of space (volume) inside the case between the bullet and the case head. This amount of space plays a large role in the development of pressure as the powder ignites. If you make that space smaller, the pressure goes up, which can cause serious damage to you and/or your gun.
But once you place a bullet in the case, there really isn't a way to directly measure that space inside. So what we have to do is take the lengths we know in order to calculate what we need inside.
We know the length of the case (the "trim to" length).
We know the length of the bullet (based on weight and shape).
We know the length of the case head (if you bothered to measure it).
WARNING: Made up numbers; do not use this as data!
If the case is .754 inches long, the case head is .394 long, the bullet is .500 long, and the OAL is 1.115, then we know that the bullet extends .139 inside the case. That leaves .221 between the head and the bullet. This is the number needed to calculate pressure.
But since we're not actually calculating pressure, just making sure that we don't end up with too much of it, all we need to do is stay at or longer than the listed OAL. That's why the OAL is listed in your reloading manual.
Note: a 124 grain FMJ from Hornady may not be the same length as a 124 grain FMJ from Remington. This is why your loading manual gives exactly the components used in testing. Varying the components, especially near maximum loads, can cause dangerous excess pressure.
Last edited by CampingJosh; 01-31-2012 at 03:16 PM..