Another way to look at it... ME is largely (but not only) correlated to peak pressure behind the pellet.
Muzzle crack is (almost totally ?) related to barrel pressure at the time of exit.

So how can you get lower pressure behind the pellet at start, but higher at muzzle exit ? Answer: piston bounce, which at it's most extreme sucks air back from the barrel, or at leastprevents the flow from the chamber. So any spring change that decreses piston bounce, can increase muzzle crack. FOr that same spring to also decrease ME, it only needs to have less energy. The two are not mutually exclusive.

HTH - JB[/QUOTE]

We'll put and agree 100%.

I have a Diana mod 6 pistol which is not exactly a powerhouse, but it cracks quite a bit and there is no dieseling. I thought maybe the recoil absorbing dummy piston eliminates the bounce and eliminates any negative pressure just before the pellet's exit which produces the crack, but then again I have other Giss type Diana rifles with obviously longer barrels that shoot faster than the pistol but don't crack.
So on this basis, I presume barrel length is part of the equation when it comes to determine which crack and which don't. I would think that at roughly 600fps, the pellet is still in the approx 15inch barrel when the compression piston has stopped moving, meaning rapidly falling pressure behind it, hence no crack.
But then again most things including cracking in airguns are the result of a combination of many variables.