Re: Bullet drop compensation dials (BDC dials)
Target turrets are usually marked in minutes of angle divided into fractions (1/8th, 1/4th MOA). These are great because once you have a zero with a given load you can write down what the setting is. As your barrel wears out or you change environment (temperature, altitude), things will change.
Bullet drop compensators only work accurate when a given load is first used in a new barrel and the compensator is calibrated to exactly what your bullet's trajectory is. Otherwise, the further you are away from the target or atmospheric conditions the compensator was set for, the greater your error will be. The error can easily be as much as a foot or more at 600 yards or 5 feet or more at 1000 yards.
I favor target turrets; easier to make changes to your written dope sheet or label stuck on your rifle's stock just below the scope so you can see it from a shooting position. And a different dope label/sheet for each load is great.