Originally Posted by uv_halo
If you look at the following video, it appears that the overwhelming mass of air is exiting straight down the bore. I believe this to be the case because the aperture formed around the projectile has a total area larger than the ports combined at any point along the barrel's length. Since this aperture is inline with the direction of the gas, this becomes the path of least resistance.
So, it remains to be seen just how much length and/or porting would be required to negate blow by and, allow for FS spin up.
One reason you don't want the rounds bouncing off the side- they may tilt. (tail wobbling side to side).
Is there a ball in that video?
I wasn't saying you do want the rounds bouncing in the barrels, just that if you over-bored to that point it should hit the sides at various points and should rotate at least a little on any rifling, even if it looses parts like a target in fruit ninja. Not that a little early rotation is worth significant damage to the round.