The shotgun barrel insert IS the barrel for the .22 (or other caliber you choose). They are rifled so the bullet spins up just like it would out of a pistol or rifle. Since it's traveling in a straight line (at least at that point) it exits the larger shotgun barrel unimpeded. I'd suggest getting one of the longer ones as you will impart greater velocity to the bullet with the longer barrel. The system achieves the same, or greater, velocity out of the insert barrel as it would out of a pistol.
One of the steps of shooting (Appleseed note here) is "Follow Through." This is when the trigger is momentarily held in the firing position prior to being allowed to travel forward and reset. I think you'd get the best accuracy with a good follow through with an insert. Additionally, the .22 insert is offset either on top or bottom so you'd need to shoot it a bit to see where it's zero'd.
WARNING & DISCLAIMER:
SELECT AND USE OUTDOORS AND SURVIVAL EQUIPMENT, SUPPLIES AND TECHNIQUES AT YOUR OWN RISK. Information posted
on this forum is not reviewed for accuracy and may not be reliable, use at your own risk. Please
review the full WARNING & DISCLAIMER about information on this
site.