HMMMMM hes getting pretty good speeds with the 16 inch barrel. over 2000 fps with that heav of a slug is going to do some damage. and the trajectory would be close to a ak round as the speeds are clost to the same. the BC is likely higher. Im not saying a longer barrel would not help but that thing is going to do some seruious damgae to what ever it hits.
Hubel can likely shed some light on what is to be gained by more barrel.