Danger space? Overlaps with caution?
Speaking of hold-over "christmas tree" reticles... most scope manufacturers also offer them in moa-based versions. Are there any reasons beyond personal preference to go for the mil version instead?
There are quite a few mil vs. MOA threads on the forum, one right below. Have a look at those.
Danger space is the range of distance over which you will still hit your target while aiming at it's center even if your scope is not set to the correct distance. For example, lets say you were shooting a 175 SMK at 2650 fps. wanted to hit a 12" target that you thought was at 860 yards. If the target was actually at 870 yards, you would still hit the bottom edge of the target because the trajectory is 6" lower than it is at 860 yards. If the target was actually at 850 yards, you would hit the top edge of the target because the trajectory is 6" higher than it is at 860 yards. JBM will calculate this for you if you enter a target height and check the "Include Danger Space" box at the bottom left.
If you put the mil ranging formula into a table, you will notice that at some point the numbers start taking very big leaps. Look at the following table:
Notice the leap in range when you go into the red areas? At that point you have gone well beyond the capabilities of the reticle. The range you calculate is not reliable and relying on it will almost certainly result in a large miss. Even in the yellow and higher green boxed, a difference of 0.05 mil in your reticle measurement will give you a distance that is not within the danger space of the box above it or the one below it. But, it will likely still be close enough that you can correct the shot easily enough. It depends on the size of your target and the ballistics of your cartridge.
In matches, you are generally ranging the target you are going to shoot, so the limits of reticle ranging for those targets is smaller than it would be in a real application where the target is often larger than the object you are ranging (torso height, for example). Danger space is a useful tool for determining the limits of ranging with your reticle. When the danger space overlaps for consecutive cells, ranging with the reticle is pretty safe.