What is MOA?
hope this finds you well. Lately when I'm not in the shop, I've had the pleasure of shooting on a piece of property out in west Petaluma, which belongs to a family friend, and in the course of shooting with my friend, we've had the ongoing discussion of what MOA is, so I thought I'd cover it here.
As you recall, I've covered a similar topic while talking about how to properly use a collimator for sighting in a rifle scope. the long and short of it is that while a laser, or a collimator will generally get you on paper, all it is doing is aligning the scope with the bore of your rifle, and essentially get you on paper, but you actually have to do some math to account for gravity and bullet drop. in most cases, we are accounting for minute of angle adjustments vertically in this case.
so what is MOA?
A Minute of Angle (MOA) is an angular measurement. A MOA is 1/60th of a degree. In general terms we usually round the number, so you might hear it said that 1MOA = 1" at 100 yards. in reality, it is 1.047" at 100 yards, but usually, this extra amount is so minuscule, that we round that number to a whole number.
Rifle scopes come in all sorts of adjustments. typically, for example, we might see 1 in 4 MOA adjustments on rifle scopes. what this amounts to at 100 yards, for instance, would be 1/4" per click, and 1" for every 4 clicks.
because we are not actually shooting in a straight line, we have to actually arc the bullet, not unlike how you would picture shooting a cannon at something. the illusion is that we are shooting in a straight line, however, if you were to put a bubble level, or angle indicator, you would see that there is an angular increase the further we are trying to shoot, and as we are adjusting our scope. in cases where a rifle has no scope, there is usually an adjustable sight which, when used in conjunction with a front sight, creates these angles.
So, what we are left with dealing is creating angles to impact at certain points in order to compensate for the lessening of velocity of the bullet combined with the effects of gravity; this is why having a known distance is essential to working out these calculations, which in modern shooting is accomplished with a range finder, because without knowing certain data, we won't know how much to adjust for.
now the further out our target is, the greater the MOA adjustments become. as we discussed, 1MOA is 1" at 100yards. at 200 yards, 1MOA is 2", and so on and so forth.
so how much would we have to adjust for a 300 yard target, where we hit 5" low? well, that depends on our ballistic data.
in the above trajectory card that I've created for our example we are going to assume that the velocity upon exit is about 2800fps, our zero is 100 yards, and the sight height is about 1.5". the bullet used for this example is a berger hybrid 155grn bullet with a ballistic coefficient of .481. our fictitious scope has 1 in 4 MOA. note: all of this information has to be known in order to calculate the point of impact.
assuming our target is 300 yards away.,our card says that the bullet drops at 300 yards by about 3.7" MOA (or roughly 11.6" inches). so we would have to bring up our adjustments to compensate. we adjust about 4 MOA or 4 whole whole numbers on the scope, and we're still a little low, say three inches. how much do we come up? well, at 300 yards, 1 whole number is 3", and 1 quarter click would then be about .75" per click. how many clicks? 3 / .75 is 4. so 4 clicks or one whole number more, and it should put us about center of the target at 300 yards.
for each distance variation, we encounter a modification of a click. so if a target is 325 yards from you, what do your click adjustments become? 325? 1MOA would be about 3.25" , if we wanted to get technical, so each click would then equal .8125" as opposed to .75" at 300yards.
knowing this, we can know that if we move one whole number at 325 yards, or 4 clicks it would be about 3.25" in adjustment, and so on and so forth.
again, we are dealing with creating angles, and there are many other factors that can be accounted for, the further out you go, such as barometric pressure, and even the rotation of the earth, depending on the direction you're facing and what hemisphere you're shooting on, but in general, for close distances out to about 1000yds, we're mainly concerned with bullet drop.
something to also consider is your wind speed, and what it is at the target, not necessarily where you are, but that is a bit more than I have time to talk about.
anyways, the more you understand minute of angle, in addition to other data which reloaders in particular have at their disposal, the easier it is to tune your rifle, and your load, and hit your target and any distance in which the distance is actually know.
anyways, I hope that this is informative and helpful. for more information, I highly recommend checking out the NSSF's video on the topic which can be found on youtube.
stay safe, and happy shooting!