MOA (minutes of angle) and MRAD (milliradian) are just units of measure for angle. In the Vortex lineup, MOA and MRAD reticles do the same thing, just on different scales. Think miles vs kilometers. The horizontal and vertical crosshairs of the reticles are marked like a ruler so you can use them for holding an elevation or wind correction. These days the industry has figured out that reticles and turrets need to share the same units, so that measurements you take with the reticle (zeroing, spotting impacts, etc) are easily dialed with turrets with no math involved.
BDC (bullet drop compensator) reticles are designed so that the tick marks on the vertical crosshairs correspond to range holdovers for a particular cartridge. If you’re shooting a good ballistic match for that reticle, you aim with the “400y” tick to hit at 400y. This a simple idea, but the ballistics rarely line up perfectly like that, and the farther the target the worse the error. The good news for BDC reticles is that you can workout the ranges where the tick marks match your load’s actual trajectory, so that “400y” tick mark may be 480y in your rifle with your load.
Hope that gives you a better understanding of what you’re looking at. Reticle design is a big can of worms, but this should give you enough to get started learning.
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