Which type of muzzleloading firearm features internals that include a nipple mounted outside the barrel at the breech end?

The correct answer is the type of muzzleloading firearm known as the Caplock, or percussion firearm. This firearm design features a nipple that is located externally at the breech end of the barrel. The nipple serves as a point for a percussion cap to be placed, which, when struck by the hammer, ignites the gunpowder in the chamber.

The Caplock system effectively improves reliability and efficiency compared to earlier ignition systems. When the hammer falls and strikes the percussion cap, the explosive reaction sends a flame through the nipple into the barrel, igniting the powder charge and propelling the projectile.

While inline percussion firearms also feature a similar design with a nipple, the Caplock encompasses a broader category that includes various models and configurations that operate on this principle of external ignition. This distinction helps clarify why Caplock is the correct answer as it describes a general type that includes specific firearms designed with this external ignition mechanism.