Originally Posted by MontanaRifleman
If you shoot a 5 shot string and get the following velocities...
3056, 3067, 3094, 3094 and 3069, Your ES is 3094 - 3056 = 38
However, the odds are very lilkely that your actual ES is smaller.
The reason being, Your chrony is accurate to plus or minus "x" fps. The shot that measured 3056 might actually be 3070 and the shot that measured 3094 might actually be 3080. For this reason, I put almost no stock in chrony measured ES's. I use actual group results to determine my best loads.
So even when the recorded velocities are collected in the same lighting conditions within a 20 minute period of time, you place no confidence in the ES?
I could understand not trusting the accuracy of the numbers; for example, is it really 3057 fps or is the true velocity 3068 fps. But I don't understand the dismissal of the variance (or spread) of the velocities on a given shot string. When I run two chronys in tandem and for five shots, the difference between the recorded velocities on the two different chronys for each shot is 15, 15, 16, 18, 16 fps, then I conclude with confidence that I've closely established the spread between the highest and lowest recorded muzzle velocity for those 5 shots.
Particularly when I receive the same type of delta between recorded velocities day after day after day over the dual chronograph setup.
I believe there is validity to the conclusion that the accuracy of the recorded velocity can vary under different lighting conditions; that a bullet whose true muzzle velocity is 3000 fps might be recorded as 3020 fps in one lighting condition and 2980 fps under a different lighting condition. But when two different chronographs spit out muzzle velocity differences within a 4 fps window, shot after shot in the same lighting conditions, I have to reach a different conclusion regarding the reality and accuracy of the ES.
I place high confidence in the variance in velocities if my deltas over the two chronos are consistently within 4 fps. I accept the fact that the accuracy of the recorded mean velocity of a particular load from day to day under differing lighting conditions may be off a bit either high or low. The MV I input in my ballistics program is the mean of the means collected from separate data sets for that same load, with some consideration given to the different ambient temperatures for each set of data, since powder burn rates and muzzle velocity would be expected to vary under differing ambient temperatures.
Lastly, and only after I've selected my final load for a rifle, I like to collect velocity over the chronographs as far downrange as possible. Farthest I've collected the velocity data to date is 990 yds. Then I tweak the BC in my ballistic program until the predicted 990 yd velocity matches the 990 yd chrono'd velocity and see how the actual measured drops compare to the predicted drops. So far they match very well.
That's how I use and apply the data from my chronographs - for better or for worse.