Baloney. Take four inches off the barrel and you'll see a wider spread and a lower velocity.
"To attempt to draw a line between the two re:RKBA is to expose a bias"
Baloney. The shotgun in use at the time was 20". The law set 18" as the minimum, not because of any bias but because scumbags like Mr. Miller were concealing them and using them in criminal acts. A reasonable restriction that you don't happen to care for.
You come along and start making this 4th grade argument about "if 18" why not 17?" Fine. Make it. But not with me.
A little lower velocity, maybe, although shotguns are not the same as rifles or handguns, they use a different sort of powder, and are lower velocity to begin with.
Secondly, spread is not affected to any detectable degree by barrel length, but rather by the amount of constriction, or lack thereof, in the muzzle end of the barrel. It's called choke. A long barrel with a cylinder choke will pattern pretty much the same as a shorter barrel with the same choke. (That's as open as you can get, it takes a special spreader barrel, sort of like a blunderbuss (the ancestor of the shotgun).
If anything the longer barrel might give more spread, because of it's slightly higher velocity with the same load. (And it would depend on the load, low base target loads would likely show the least difference, with higher speed, and higher recoiling, hunting or defense loads showing a bigger variation in velocity and spread. In any event the difference would be very small, as it's a secondary effect. The primary effect is the choke.
were concealing them and using them in criminal acts
Then ban the concealment and use in criminal acts - just as the Parker verdict observed. Prohibition of mere ownership is not warranted ... especially when modern militaries have discovered that sub-18" barrels ARE particularly useful.