If it is a 2nd focal plane scope the BDC will only work at a specific magnification there should be a couple marks on your scope where the magnification is. If you look in the manual one will be for 30cal bullets going a a rough velocity and the other will be for like 6.5ish cal bullets.
you will have to have your scope set to that magnification every time you use the BDC. The only thing that will stay the same when you adjust magnification is the center crosshairs. So let’s say 7 power is your magnification lever for your bullet size and velocity if you use the BDC at 3x it will not work correctly.
Keep in mind that every bullet and velocity is different and the factory mark on the scope for your bullet might not be perfect but for hunting situations under 400 yards as long as you have it on the right magnification you will be ok.
^ this.
No one has any business shooting further at game than they have validated / practiced before hand at targets....