Most of you will know that for the vast majority of shooting I suggest a 4% or smaller Extreme Spread (ES), ie the difference in velocity between the fastest and slowest shot, compared to the average velocity.... I've written numerous posts about why I use that number, but perhaps it's time I looked at it from a different perspective.... Most of my guns are tuned to shoot in the low 900s, and I seldom use anything but a round-nosed pellet, so that is what I have used to develop my 4% ES idea.... Let's look at what happens if we use a pellet with a much different BC.... or use a vastly different velocity....

I used three different velocities, 1000, 750, and 500 fps.... Then I used three different ES, representing 2%, 4%, and 10%.... I used three ranges, 20, 50, and 100 yards.... Lastly, I used three different BC's, of 0.010, 0.030, and 0.050....This should cover nearly all the shooting conditions except 10M competitions, and I'll even comment about that later.... I used ChairGun's Vertical Stringing Widget to calculate the total height of the string due to velocity change for each combination.... The table below gives the results for each velocity, and the values in red exceed 1 MOA vertical dispersion at that range.... I used that as an arbitrary value, but in fact any shooter who expects MOA accuracy from his gun will want the vertical stringing from velocity to be maybe half that value, meaning an even tighter ES to attain that at the range he is shooting....

1000 fps



Note that at these velocities, the BC doesn't make a lot of difference until you push out past 50 yards.... The fact that the vertical stringing is worse with a wadcutter at 100 yards doesn't matter to us, as we would never use them at that range anyway.... With decent pellets, a 2% ES results in about a 2/3 MOA vertical stringing at 100 yards, about the maximum most shooters would accept.... The same thing applies to a 4% ES at 50 yards, which is why I have always used that.... At 20 yards, a 10% ES works out the same, about a 2/3 MOA dispersion....

750 fps



When we drop down to 750 fps, we need to tighten up the ES to stay within the 1 MOA vertical stringing.... In fact, at 50 yards, a 2% ES is about the same as a 4% ES at 100 yards when you are shooting at 1000 fps.... A 10% ES will no longer hold a 1 MOA vertical dispersion at 20 yards.... You can also see the effect of a poor BC on the results at closer ranges....

500 fps



When we drop down to 500 fps (as is the case with non-PAL airguns), the required ES tightens up even more.... In fact, the only combination that is under 1 MOA in the chart is a 2% ES at 20 yards.... A 2% ES is about 1.5 MOA stringing at 50 yards, and a 4% ES is over 1 MOA at 20 yards.... It is pretty obvious that for non-PAL airguns, you want to keep the ES as small as possible, even at the relatively close ranges they are typically used at.... If you are shooting in 10M competition, the velocity is typically about 500-600 fps, and wadcutters are used.... This means that if you have a 2% ES, the vertical dispersion is about half what is shown in the table above, or about 1/16".... Therefore, you won't want anything over a 10 fps ES at the most....

In my next post I hope to present some graphs showing the maximum ES vs. range for a few common examples, to obtain a given MOA of vertical dispersion.... I hope you find this information useful....

Bob