Biscuits wrote:The problem with BDC is they only work well with a fixed ammo type. Put in subsonic ammo, the bullet will impact in one place. Put in high velocity, it will impact in a different place. A scope you can adjust easily accounts for that.
Nope, BDC's work exactly the same regardless of chambering or ammo.
I use 223BDC on everything from .22LR to .303 and .44 Magnum, makes zero difference. The holdovers will generally be at specific distances, so unless you shoot at targets that happen to be at those specific distances the BDC is not relevant anyway. What you need to know is where you need to hold on the reticle at any distance.
The advantage of a second focal plane scope is that you can alter the BDC as well. My BDC has holdovers at 1.7, 4.4, 7.6, 11.4 and 15.4 minutes at 18-power. Winding the zoom back to 9-power doubles those to 3.4, 8.8, 15.2, 22.8 and 30.8 minutes. Coming back to 4.5-power quadruples them. You can print yourself a DOPE chart and use the zoom in conjunction with the BDC to use it in virtually any situation. For precision shooting you would avoid using BDC holdovers at all, dial the elevation to give you a dead-on hold to avoid adding reticle cant.
When you have worked up or decided on your load of choice shoot it at various distances and measure the trajectory at several points - with .22LR and other low-velocity cartridges, 50m, 100m, 150m, and 200m are good, with flat-shooting high-velocity cartridges 50m, 100m, 200m, and 300m are good. Also measure your muzzle velocity if at all possible. With the trajectory and the muzzle velocity you can calculate the BC of the bullet (ballistic coefficient is simply a numerical value given to how rapidly a bullet loses velocity due to air drag).
Then you can calculate your trajectory curve and determine the distances at which it matches the stadia on your reticle.
My 223BDC mentioned above for example, with a 100m zero with CCI Standard Velocity puts me on at 110m, 125m, 140m, 160m and 175m. Winding the zoom back to 9-power puts them at 120m, 145m, 175m, 210m, and 240m.