Justus has summed it up with as good an explanation as I've seen.
The short effective length of a freak insert will only be moderately effective in your case. You will be going from a small bore at the start of the barrel to a larger bore for the majority of the barrel and then back to a rubber bumper to affect the spin. You are probably better served by the single full length bore size of the lapco barrels when using an apex attachment.
Except for the Automag in front, its usually the man behind the equipment that counts.