Here is why it is possible to have a tight ES shoot poorly, and a wider ES shoot fine...
A sketch from my archives...
When the main shock wave (the wave which is initiated by the ignition of the powder, and when then travels at approximately 18,000 fps through the barrel to the muzzle, then back, then forth, etc.) has reverberated away from the muzzle, there is still a pattern of vibration at the muzzle. The muzzle is "drawing" a pattern in the air while the bullet is still travelling toward the muzzle to exit the bore.
If the muzzle is moving through a "straightaway" of this pattern, you can actually have a tight ES and the bullets will "string" on the target.
Conversely, if the muzzle is at one end or the other of this vibration or "whip" pattern, it (the muzzle) will be slowing down and nearly stopping before it reverses direction. There is a time window here which will allow bullets being released all along it to be essentially in the same point in space as they begin their flight.
So, you could actually have an ES of 25 fps with, say, a 175 grain Sierra Matchking from a .308 win, and there would be less than 1 MOA of space between the 2625 fps and the 2650 fps shots at 1000 yards--provided that these two bullets were released on an optimal point along the vibration pattern.
And of course you could have an even tighter ES, say 10 fps--and if these bullets were released along the "straightaway" as shown in the sketch above, then they're going to have a more angular relationship to one another, and print farther apart at 1000 yards--even though the ES was smaller.
I do agree that as we approach the optimal load zone, the chronograph numbers tend to tighten up. But that doesn't ensure that we have the bullets exiting the muzzle on a good (near static) node of the vibration pattern.
Which is, to repeat myself, why the target is always the final arbiter. Otherwise, we could save some target paper, and let those guys working the pits sit back here with us while we all see who can get their Oehler 35's (soon to be out of production, by the way) to spit out the purdiest string of numbers for the trophy. [img]/ubbthreads/images/graemlins/wink.gif[/img]