Quoted:
These are not effective for any long range use as the scope is sighted in to the receiver and not the barrel.   The barrel of your weapon simply fits into a slot and the magazine cap or mag tube coupling holds the barrel on.   So, the barrel can move when being fired as it is not pressed, pinned or threaded on.   For optics to be sighted to the barrel they have to be mounted to the barrel as with the Cantilever mount, which as you know is fixed to the barrel.   When an optic is mounted to this type of mount is stays true.   The thing is that cantilever mounts are on fully rifled barrels only.

The reciever type mounts like you have or even mounting rails on the receiver are OK for optic for most shotgun shooting which is at close range.  The length of the barrel has no baring on this so getting another barrel is of no use..............

Some will disagree but I firmly state that optics have no place on an HD and are simply more mess to get in the way when you need to rapidly deploy the weapon.  First pick for any fighting shotgun is a bead sight, then rifle sights.   That is all.........and as for GR's, same principle as the receiver mounted scope, the rear sight is on the receiver and the front sight is on the barrel so the they are not consistantly accurate like rifle sights which have both sights on the barrel.   Other will dispute this but how can they dispute when it is the design of the weapon that causes this.   So they are simply stating an opinion that is not supported by facts, the fact being that the barrel is not fixed so the POI can change as you fire the weapon.  It will also change more when you have removed the barrel for cleaning and reinstall it.   This is why custom slug gun makers are pining the barrel on their custom builds when they feature any receiver mounted sights.