MD the difference is that the ER Shaw barrel is thicker than the Factory rifled slug barrel and you had to get a different fore end to go on the weapon. Now if you watch Gunbroker you can come across some deals on Benelli slug barrels. I got my slug barrel off of there for 250.00 practically brand new. Good luck
They will swap out. If you can find one of the earlier model ER Shaw barrels offered for the M1, it has the cantilever barrel but you will also have to get the rifled slug fore end for that gun.