Bullet Drop Compensators (BDC) are not new. You can see them on rifles as soon as we started using iron sights for aimed fire. The concept is straight forward, and they have worked very well. A BDC is designed around a specific load in a specific rifle under specific environmental conditions. The closer you are to those conditions, the more accurate the BDC will be. The theory is simple, use match combined with downrange feedback to develop the proper sight setting to get hits at various ranges.
With iron sights this was usually accomplished by a dial such as the M16A2 or a slider as seen in the Kalashnikov variants. In optics, the stadia lines are etched to represent the proper hold at that distance as seen in the Steiner 536, Leupold MK6 series and the Trijicon ACOG line. In a military context, the BDC makes sense. Standardized rifles and standardized ammunition make the BDC easy to calculate and confirm. This allows the user to quickly determine a hold after the range is estimated and engage. Under these conditions, the round will land, provided a solid shot process (See TC 3-22.9 Rifle and Carbine) at or near the target. This makes training simpler due to no having to train exterior ballistics across the force.
Where BDC’s fail though, is when any of the conditions are changed. The US Army found this out the hard way with the deployment to high altitudes in Afghanistan, the transition from M16A4’s to M4’s and the switch from M855 to M855A1. The reticles inside the widely fielded RCO went from calibrated holds to guesses with each change. Having all three conditions changed meant the Soldier really had a zero point, and stadia at random points along the reticle. With the use of a ballistic calculator, you can work through getting your BDC back to known points provided you have some basic information. If you have the conditions of where it was set and/or the distance in MOA or Mils between the stadia, you can easily apply the new data and have the new hold information.
There are some who say its close enough, but that is not acceptable. If the stadia are supposed to represent 400 yards or meters (yes it matters) then it should truly represent that. If the bullet is flying to 360 yards vs 400 the target will not even know they are being shot at. It makes it about the same as a red dot optic where you know the bullet will land somewhere under the aiming point. If you are lucky enough to see an indicator of the strike you can adjust. If not, you will continue to miss each shot. This destroys confidence in the rifle, the optic, and the user’s skills. Nothing is worse than bad data.
There is nothing wrong with BDC’s provided you account for their specificity. They are either right, or wrong. If they are wrong, do some work to get the right data applied. If you have an optic with a BDC set for something other than your combination, do not fret, do some research, and make it work.