Now when you look at the 25x magnification, you can see the target looks much bigger, while the reticle has stayed the same size. The 30" IPSC is now 11.6 mils in size. So, we know that in order for mil-based measurements to be accurate, 1 mil must subtend exactly 3.6" at 100yds. As we see here, the linear distance that 1 mil subtends with SFP scopes is dependent upon which power you are on. Typically, the power at which they will subtend correctly is calibrated at the factory to the max power setting. You can also use a bit of math to figure out the correct subtension at the other powers. Setting the power ring on 1/4, 1/2, 3/4, and the math is relatively simple. However, you can never be sure if you are precisely where you need to be on the power ring to make your calculations correct. You also can not be sure if the scope is correctly calibrated at those powers unless you verify it. The primary reason for wanting the reticle to subtend correctly at various powers is so that when you apply a wind hold or hold-over, it will be correct. If you use the above example, you can see that if you held for wind or elevation using the reticle at 5x, you would not be able to use that same hold at any other magnification. Another problem you encounter is accurate range estimation. If the reticle does not subtend the correct distance, then your calculations will be off and will result in the wrong distance to target. There are those among us that can do the math and dial the scope to precisely the correct position to make holds subtend correctly using SFP reticles, however there are none that can do it quickly and accurately under pressure that I'm aware of. With SFP reticles the reticle itself covers a different amount of area on the target depending on the magnification. In the illustration above you can see that at 5x magnification the reticle covers more area of the target on low magnification than it does on high power. If you are on low power, then chances are that the target is close enough that you will not need to worry about it covering too much of the target. The most effective method of using a scope with the reticle in the second focal plane is to rely on your turrets to make the appropriate adjustments. When dialing the turrets you remove the error involved in using the reticle for holds, as your turrets do not care what power the scope is on. One click is one click, regardless of power. So if you need to come up 10.2 mils, you simply dial 10.2 mils into your elevation turret and hold dead center with the crosshair. There are cases when this does not hold true, such as shooting in areas with very rough terrain. It has been proven that on very long shots, updrafts and downdrafts on the face of cliffs or mountainsides can cause a bullet to rise or drop. This falls into the wind reading category and is pretty much all instinct. It's tough to dial instinct. The windage is a little trickier than just dialing it. Obviously when you come up with a firing solution you will be given a windage adjustment to make. The problem with this is that the wind is always changing. Wind holds are every bit as much instinctual as they are precise calculations. As I pointed out above, it is very hard to dial the "instinct" your brain is giving you and fire the shot at the correct moment. One method to overcome this is to "ambush" the wind. With this method you simply wait for the wind to match your dialed adjustment. This idea of waiting for the condition to match the shot doesn't sit very well with the tactical shooting crowd as they are often forced to shoot in any condition due to time pressure. This is especially true for a professional marksman such as a LEO sniper. When the shot needs to be taken, it must be taken. He must make his shot match the conditions, n o t t h e o t h e r w a y a r o u n d .
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