May 30th, 2013, 10:36 PM
Cartridge Overall Length (COAL) is a balance between two primary things,
Bullet shape and dimensions can effect how much volume there is inside the loaded cartridge. Hornady rifle bullets, for example, usually take up more space inside the case as compared to other brand bullets of the same weight. Therefore, Hornady bullets that are seated to the same depth as another brand bullets, of the same weight, will produce higher pressures and velocities. Some bullets with different shaped ogives (the area where the bullet's diameter starts to become smaller) will cause you to have to adjust the seating depth one way or another (longer or shorter) in order to optimize accuracy. Rifles with detachable magazines can force you to have to seat the bullet deeper so that it will clear the front of the magazine.
If a manual recommends a longer COAL than another manual, using the same bullet weight, then the longer cartridge will, quite often, have more mass seated inside the case or the ogive is shorter, or both.