Hey rockytop65,
This is going to STING a bunch of our readership so watch me get flamed by some; pay attention to those that agree!
Just how important do you think ES and SD are to real accuracy and precision? I'll start by asking why it is most dope given in FPS instead of a more meaningful percentage of AV (Average Velocity) and TOF (Time Of Flight) which are much more representative of the comparables. I know that quite a few readers know a great deal more than I do about Long Range Shooting but I do know my External Ballistics from lots of through study. So here is the story: The lower the AV, the longer the range, the longer it takes for the round to reach the target; TOF. Therefore, a small variation in velocity will make a bigger difference in the Point of Impact (POI), with faster shots hitting higher and slower shots hitting lower. This is called vertical dispersion, or stringing.
The higher the velocity, and the higher the Ballistics Coefficient (BC), the less critical the ES is. Most of us would be happy if we could shoot groups of 1 MOA, but few of us can do that regularly. Consistent velocity (0% ES) is unnecessary, and so we need to come up with a degree of ES that makes no difference to our shooting system type.
I believe most of us would accept a vertical dispersion of 0.5 MOA as being completely lost within a 10 shot group size, and therefore 99.9% acceptable. Remember, we are talking total MOA spread here. The actual possible "miss" distance is half that, only ¼" high or low at 100 yards; I hope that has most of your attention. I'm going to go to a different shooting paradigm so you all can suspend disbelief for what I'm going to write about next. We will go Subsonic at short ranges!
Stay with me...as the velocity increases, at any given range, you can get away with a larger ES and stay within that 0.5 MOA vertical change in the POI. At 900 fps, with a 2% ES, out to 55 yards you will not see any error caused by that 2% (18 fps) velocity variation. The same applies with a 1% ES (9 fps) at 100 yards. However, at about 700 fps that same 9 fps ES (1.3%) would cause that same 0.5 MOA change in POI at 50 yards.
With bullets that slow down faster and/or start out at a lower MV, the ES becomes even more important. At 30 yards, at 500 fps, a 1% ES will cause about a 0.5 MOA vertical string. Increasing the velocity to 700 fps means you can double the ES to 2% and still not see the effect.
There is something else to consider here, to confirm the idea that we don't need to obsess about the ES. The very best .22LR target ammunition (with a muzzle velocity of about 1050 FPS) has an ES of 2-2.5% over a box of 50 shells. You would think that since we can easily do better than that ES you would see when shooting a Target .22 rimfire at 100 yards. The fact that we can't, confirms the idea that a 0.5 MOA vertical dispersion is lost within our actual group sizes.
So, velocity is the prime factor in determining the projectile's trajectory. But there is another factor besides the trajectory in determining if you hit your target, shoot over the top, or hit low. That is the distance to the target, and how close you are in estimating that. You may know the trajectory of your round, but what happens if you miss estimating the range by just a small amount, say 2% (2 yards at 100 yards) So just what is important? You know...
What about the effect of wind? Let's say we allow for the wind, but guess the wind speed wrong by only 2 mph? Lastly, how about if we are using good loads, but our ES is 4% instead of smaller as we might like?
It should come as no surprise to you that the biggest cause of missing your target is the wind. If you estimate the windspeed incorrectly by only 2 mph, you will miss your target by over 1 MOA at 55 yards and beyond. That is a ½ Inch error at 50 yards, from misreading the wind by just 2 mph!
What about that 4% ES you have? It turns out to be less important than a 2 mph wind, but could cause a miss of 1 MOA (3/4 Inch) at about 75 yards. Tighten up the ES a bit and it becomes even less important by the time you get out to 100 yards, just a 2 yard error in ranging is about a 1.6" error high or low in the POI.
i was going to talk about typical Chronograph error margins but I'll just say I use a LabRadar to get Average Velocities all the way to the target and calculate real BC for every projectile and weapon combination.
i think the bottom line is this is way too long already! I thank those of you who got to this point in an over-long post!
My point is basically use AVERAGE Percentage and not FPS based ES and SD so that you have more time to actually shoot and learn to make the wind your friend!
shootski
