A muzzle loading firearm has a sound suppressor mounted on the barrel. Attachment of the suppressor to the barrel may be permanent, as by welding, brazing and adhesive bonding. A guide tube inserted into the suppressor is used to provide a continuous path from the open end of the suppressor to the muzzle end of the barrel to permit loading of propellant charge and projectile from the muzzle end. The guide tube isolates the chambers of the suppressor and prevents grains of the propellant charge from being lost in the suppressor as they are poured into the barrel. The tube also protects the internal components of the suppressor from damage during loading with a ramrod.