Bullet Drop Compensators (BDCs) have been around in various forms for a long time. The idea is that if the trajectory of a given ammunition or load is known then the compensating "ups" for a given range can be preset into the sighting system. This would be true only if you happen to be shooting at the exact "standard" conditions of the predetermined trajectory. You seldom are. There is a very high likelyhood that the temperature, barometric pressure, and your muzzle velocity (among other factors) will be different. Few BDCs have any allowance for this and those that do are not precise enough to give you accurate trajectory compensation. To 1000 yds the problem is not that complicated. Learn your ups in 100 yds increments and how to adjust for intermediate distances. Also, learn the compensations for Temp and BP. Beyond 1000 yds. you will need computer generated tables and hand calculations, or you will need a computer. Maybe one of these days there will be a sight BDC that will compensate for temperature, barometric pressure, muzzle velocity, and the other factors. I have not seen it, yet.