Originally Posted by Spider!
A few of thoughts to add;
On an over-bored barrel the round should at least be bumping into the sides as it goes down the bore. If not, I would use that barrel anyway. When it bumps, it will be bumped to twist with the rifling. I don't see how you would not get some rotation.
Any rotation from the barrel is a jump start and should be a good thing if the round can stand it. Too much rotation (more than the angle of the fins) should be wasted unless you have a muzzle condition, which is next...
Porting and barrel length should matter (if any of this does). On a short bull barrel (no porting) the gas escapes forward of the round, which should at least delay the establishment of air flow over the fins, if not push them the other way. With the right amount of porting and barrel length, the round can exit with little or no blow-by.
Yes? Da? Si? o no?
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).