The bb should travel within an air cushion, travelling along the top of the barrel would cause more uncontrolled spin the same as over hop. This is meant to be a worse problem in 6.01 tb barrels. I have also read the theory that they travel along the top of the barrel the problem is nobody seems to have done any real experiments to prove which theory is correct. I got rid of my prommy 6.01 because I found my pdi 6.05 and orga 6.23 to be better, might have to get a new one and try an experiment of my own.