I'm going to make the numbers up for simplicity of understanding, but I think it goes something like this:
Lets say 20" barrel will give a 18 inch wide pattern at 20 yards. That's 9 inches for every 10 yards.
So, a 10 inch barrel would give you an 18 inch pattern at 10 yards and a 27 inch pattern at 20 yards.
Same pattern, but the 10" barrel starts opening up 10 inches quicker than the 20 inch barrel....
DEERP... That's confusing. I think a MSPaint picture is
Again, totally made up numbers.... I'm just trying to say that I think there is some sort of correlation between the length and spread; that it's probably some sort of ratio/equation. Also, that with #8 shot, the pellet count is sufficiently high such that the spread doesn't matter too much when it comes down to busting clays. The odds that one (one is all it takes) will hit the clay within 50yards is pretty high regardless of barrel length...Could be totally wrong. I recall reading this somethere. Might be an interesting test for someone to try! hehe.