You will need to choose between an MOA and MIL (MRAD) system of measurement when choosing your scope.
We all tend to relate things to known standards. We cannot measure things without reference to something we know and understand, like an inch or centimeter. In shooting, a measurement like inches can be helpful, but for shooting at a target, we tend to use an angular form of measurement.
MILs/MRAD and MOA are angular measurements. If we want to describe a distance or size, we need to be able to imagine what that means in our minds. The same goes for shooting, as well as any compensations we need to make out at distance. This is critical when working with others, as you need to tell them precisely what to do. For example, if you are calling shots to direct someone else as a spotter, then you must be able to tell the shooter exactly what to do, like “shoot 2 MOA left” to compensate for the wind at the target. This is a precise distance that they can measure in their scope reticle to change the point of impact.
This use of measurements can be confusing, as it’s used in discussion, the use of reticles, scope turrets, and in calling shots. So it's important to learn what makes sense based on your mission. If you plan to hunt in the U.S. only, you will likely focus on MOA, whereas if you plan to shoot some competitions internationally, they may use MILs instead. This means that everyone in the community you will be shooting with will be using this measurement “language,” and so you need to be able to “speak” it as well.
The same goes for different training schools you may attend, especially since any that are more military focused may use MILs and meters for measurements, whereas hunting focused schools use MOA and yards.
All you need to do is pick the mission, and your community, and then understand what language they use. Also take this choice (MOA vs. MILs) into account with equipment purchase, especially in scopes. The good thing about other tools is they usually can accommodate these different measurements. Ballistic software will allow you to pick your system, as do good range finders, and other accessories.
If you understand the basic differences, you can work with anyone and convert between systems as required, and this can be very helpful when working with new people, especially new spotters.
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Be aware of your mission and what units they use. Be prepared for a mix up between inches and angular measurements, and know how to speak as many unit languages as you can.
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