What part of a muzzleloader is referred to as the 'nipple'?

The term 'nipple' in the context of a muzzleloader refers specifically to the area where the gunpowder is ignited. This component is a small threaded fitting that accepts the percussion cap, which, when struck by the hammer, ignites the gunpowder in the chamber. This ignition process is critical for the firing mechanism of a muzzleloader. Understanding the function and location of the nipple is essential for safety and proper handling of the firearm.

The other options refer to different parts of the firearm: the storage compartment for bullets is typically referred to as the magazine or loading port, the metal part that holds the projectile is known as the chamber or bore, and the component that holds the firearm stock would be related to the receiver or action, but none of these serve the purpose of igniting the powder, which is distinctly the role of the nipple.