craigd: The pellets actually don't clear the muzzle the at same velocity if any choke is present.
There's a velocity differential at the start.
Even if there is no choke, aerodynamic drag will spread the shot cloud out and introduce speed variations because the pellets initially are not flying in 'clean' air. They influence each other until they get spread out.
Hard pellets deform less, which results in less drag variation pellet to pellet. That's why they shoot tighter and string less.
The effect of shot stringing is both under and overplayed in the literature.
I did the math once and found that on a hard angle trap target at 27 yards handicap, during the time of shot cloud passage the target 'only' moved 3 inches. That's not much, right?
But consider that the target is only a bit over 4 inches wide. It moves almost a whole target diameter during cloud passage!
Does this matter? Not a hell of a lot, but it does reinforce the idea that things happen in 3 dimensions and rather makes the photos of Don Zutz holding a clay target over supposed 'voids' in a 2 dimensional pattern look a little foolish. The target does not experience that 'hole' unless it's glued to the pattern board.
Your nice neat 30" pattern circle becomes a tilted ellipse with eccentricity that increases with both target speed and target deflection, when viewed FROM THE PERSPECTIVE OF THE TARGET.
In the final analysis, it's what the target experiences that matters.
