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Standard Deviation (SD) and Extreme Spread (ES) are two components that may be overlooked for beginners and even intermediate shooters. This may be because a large part of rifle shooters are hunters.
Most hunters will not be taking shots past 300 yards, if that. My dad told me in his many years of hunting he never took a shot over 100 yards. If this is the case for most hunters, then there is no need to know SD and ES. But, for those wanting to shoot long range with consistent first round hits, this is another important part of the puzzle.
Standard deviation is a number used to tell how measurements for a group are spread out from the average. A low standard deviation means that most of the numbers are close to the average. Extreme spread is the difference between the minimum and maximum feet per second (fps) of a string of fired rounds.
Let's start with a formula. This formula will only work with a milliradian scope. Meaning, a scope with matching redical and turrets using mils.
(1/100 yards) / 3 x (number of clicks) = Distance round will move in INCHES
1 click = .1 Mil
This formula tell us how many inches our bullet will move per .1 mil or 1 click on our scope.
Now, let's get back to SD and ES. I have personally seen 60+ fps difference with factory ammunition. And, on average, about 30 fps ES. Let's see how this affects our bullet.
In this example, let's use a .308 with a 175 grain SMK moving 2,686 feet per second.
At 1000 yards, we would dial up 9.4 mils of elevation. With an ES of 60, if we change our fps to 2,626, we would dial up 9.9 mils at 1000 yards. Let's run that through our formula from earlier.
1/100 of 1000 is 10. Move the decimal over two places. From 1000. to 10.00
10 / 3 x 5 (the number of clicks between 9.4 mils and 9.9) = 16.66 inchs difference between two shots. If you are shooting a 1 MOA target at 1000 yards (10.47 inches) you have missed by over 6 inches due to ES. With 30 fps difference or 9.6 mil dial up
we have 6.7 inch difference. This will most likely be a miss as well. If we are holding center target, you have 5 inches below center before a miss. With the bullet drop of 6.7 inches difference, that's a miss. Don't forget we have yet to calculate for wind. We'll save that for another time.
Let's look at this from another angle. The Industry standard for a good shooting rifle is 1 MOA or 1.047 in at 100 yards. Let's say you have a 1 MOA rifle. At 1000 yards with all environmental factors set aside, you would shoot a group size of roughly 10.47 inches. This is assuming you have perfect ES. But if you add the error of 30 fps for ES from the example above, that's a 17.17 inch group. Remember, we have set aside the environmental factors, so those can increase our chances of a miss.
So, with all this knowledge of SD and ES it would seem almost impossible to hit such a small target. But this is something we can minimize. We can mitigate this error with quality ammunition. You will have to test multiple ammunition brands and bullets. However, you will still be at the mercy of ammunition makers' quality control. In my opinion, the best option is reloading your own ammunition. You will be able to focus on the specifics of the ammo you build. You can build a round specifically tailored to your firearm down to the last detail. Type of case, type, and weight of the bullet, bullet speed, seating depth and with an incredibly low, single digit SD and ES.
The Overwatch
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