[ThunderCactus]

You just gave me a good idea!
If a BB is properly balanced, it has no choice but to spin around it's center mass, but perhaps it's not always round, and the center of mass changes when it goes through the hop rubber.
The added pressure of forcing the BB to skid through a short section of the barrel wall may deform the BB, or change the balance just enough to cause it to move its center mass ever so slightly.
That would account for better accuracy and range on the low pressure flat hops, but I'm not entirely sure just how it would affect the magnus effect.

But the skid mark could change the grip of the BB, causing it to fly up sooner. And applying less hop obviously gets rid of the hump, but having a smoother surface would cause the magnus effect to show later in the flight path and come on more gradually, giving you a flatter trajectory and more range.
The magnus effect gets stronger as the forward velocity is reduced, a reduction in surface smoothness could cause the BB to slow down faster and get more lift...
I like the second idea better lol