Muzzleloader firearm system

A muzzleloader firearm system, comprising:

• a gun barrel, having a generally cylindrical bore with a breech end, and a plug retaining structure at the breech end;
• a breech plug, installable and removable from the breech end, having an interlocking structure configured to interlock with the plug retaining structure via axial rotation of less than about 360 degrees, having a primer recess disposed at a rear thereof, and having an ignition port extending through the breech plug, configured to direct ignition gasses from the primer recess to a combustion region within the breech end;
• a movable action member, configured to move between an open position and a closed position adjacent to the primer recess;
• and
• primer retention slot, disposed at a forward end of the action member, configured to removably accept a primer, whereby the primer mates with the primer recess when the action member is closed, and is retracted from the primer recess when the action member is opened.