Hmmmm, 2016 going into 2017 and people are still shooting smoothbore designed "Foster" slugs thru their rifled shotgun barrels and complaining about the barrel getting leaded up.....
Here's my experience with the Lee Drive key slug fired form a Remington 870 with rifled barrel;
http://castboolits.gunloads.com/show...z-load-success!!
As for your Mossberg rifled barrel. Is It ported? These will do a nasty job on the wad if shooting Lee slugs which will affect accuracy.