To range with a mil-dot system or with a reticle that has MOA hashmarks, you need to estimate:
- the size (typically height) of the item you are ranging
- how many mils/MOA that target subtends.
In each of these there is an error.
For example, maybe your best guess for the bird's height was 6" (therefore you should use that!), but what if in reality it was 5.5" tall? That's an 8% error, which will feed through your calculation and produce an 8% error in your calculated range.
In your example, working backwards from the figures you provided, I think you determined that the bird was 4 hashmarks high (which would be 0.4 mils). To what degree of precision are you able to gauge the height of the birds in hashmarks? What if the bird was 3.9 hashmarks high, or 4.1 hashmarks high? An error of a tenth of a hashmark out of 4 hashmarks is a 2.5% error.
Your calculated range estimate will include the errors that go into the calculation. With the error estimates I've given, it's pretty easy to imagine that you could fairly easily have a 5% error in your calculated range (so about 30m of error), and it's quite possible that the error could even be as high as 10% (in which case you'd have about 60m of range measurement error).
My figures are just my own guessed of what the errors might be. You probably can make a better estimate of reasonable errors you'd expect in your situation. For example, if I was on a rifle range and was looking at a targetboard that I *know* to be 4' square, then the error in the target size is 0%, since I have a way of knowing *exactly* what the size of the target is. And in measuring such a large item, a tenth or two tenths of a hashmark are (percentage-wise) much smaller errors.
Let's say for the sake of argument you wanted to shoot a 6" high bird at about 600 metres. That's an enormously difficult target to hit in field conditions, BTW - the target is perhaps 0.5MOA wide by 1.0 MOA high. You'll need a pretty good target rifle to be able to shoot 0.5 MOA groups at 600 metres (though this is quite achievable). Let's say you got pretty lucky and your factory .308 is able to shoot 0.5 MOA groups at that distance, perhaps using handloads you make with Sierra 175 Matchkings fired with a muzzle velocity of 2600fps.
You can run ballistics and have a table handy - I just ran it through JBM and got this:
Calculated Table
Range Drop Drop Windage Windage Velocity Mach Energy Time Lead Lead
(m) (in) (MOA) (in) (MOA) (ft/s) (none) (ft•lbs) (s) (in) (MOA)
600 -131.4 -19.1 40.4 5.9 1533.6 1.374 913.8 0.985 173.4 25.2
605 -134.4 -19.4 41.2 5.9 1525.8 1.367 904.5 0.996 175.2 25.3
610 -137.4 -19.7 42.0 6.0 1518.0 1.360 895.3 1.007 177.1 25.4
615 -140.5 -19.9 42.8 6.1 1510.2 1.353 886.1 1.017 179.1 25.4
620 -143.6 -20.2 43.6 6.1 1502.5 1.346 877.1 1.028 181.0 25.5
625 -146.8 -20.5 44.4 6.2 1494.8 1.339 868.1 1.039 182.9 25.6
630 -150.0 -20.8 45.3 6.3 1487.0 1.332 859.1 1.050 184.8 25.6
635 -153.2 -21.1 46.1 6.3 1479.3 1.325 850.2 1.061 186.8 25.7
640 -156.5 -21.4 46.9 6.4 1471.7 1.318 841.4 1.072 188.7 25.8
645 -159.9 -21.6 47.8 6.5 1464.0 1.311 832.7 1.084 190.7 25.8
650 -163.3 -21.9 48.7 6.5 1456.4 1.304 824.0 1.095 192.7 25.9
655 -166.7 -22.2 49.6 6.6 1448.7 1.298 815.4 1.106 194.7 26.0
660 -170.2 -22.5 50.4 6.7 1441.1 1.291 806.9 1.117 196.7 26.0
665 -173.8 -22.8 51.3 6.7 1433.6 1.284 798.4 1.129 198.7 26.1
670 -177.4 -23.1 52.3 6.8 1426.0 1.277 790.0 1.140 200.7 26.2
675 -181.0 -23.4 53.2 6.9 1418.4 1.270 781.7 1.152 202.7 26.2
680 -184.7 -23.7 54.1 6.9 1410.9 1.264 773.4 1.163 204.8 26.3
685 -188.5 -24.0 55.1 7.0 1403.4 1.257 765.2 1.175 206.8 26.4
Your mildot-ranging said the target was 666 yards away which is 605 metres. The ballistics table above tells you that you need to add 19.4MOA (the figure I've made dark green) to your 100m elevation zero (if your scope knobs are marked in tenths of mils, you can get the printout in mils too). Scope knobs usually have clicks in quarter-minutes so you'd choose the nearest quarter which would be 19.5 MOA above your 100m zero.
Your GPS says the distance is 626 metres. The table says for that distance you'd need 20.5 MOA (red figure) of elevation. Note that this 20m difference in range requires about 1MOA difference in elevation (at this distance, 1 MOA is about 6.5 inches).
For the sake of argument let's assume that the true distance is 626 metres. When you make the shot using the correct elevation for your measurement of 605 metres, the centre of your rifle's 1/2-MOA (assumed) group will be about 6.5" below the centre of the bird. This is a big enough error that it'll pretty much spoil your chances of getting a first-round hit on the bird.
By the way, I think a "mil'd" measurement of 605 metres vs. a "true" distance of 626 metres is an EXCELLENT result.
