An ammunition system and associated ammunition for firearms, particularly for muzzleloader firearms, having a predetermined amount of propellant charge housed in a casing, and a separate projectile having a sabot. The casing has a cap sealing the casing muzzle end such that the cap and the sabot provide seals to trap propellant gases within the barrel behind the projectile upon firing. The cap top end and sabot bottom end interlock in mechanical communication with one another upon firing allowing the projectile exiting the barrel muzzle end to be responsive to the barrel rifling. The barrel breech end has a chamber bushing for receiving the casing and a tapered muzzle end to prohibit the projectile from entering the casing.