What does the term "nipple" refer to in a percussion muzzleloader?

In the context of a percussion muzzleloader, the term "nipple" specifically refers to the threaded piece that is designed to hold the percussion cap in place. The nipple is an essential component of the firearm's ignition system. When the hammer strikes the percussion cap, it creates a small explosion that sends a flame down into the barrel through the nipple, igniting the gunpowder and ultimately firing the projectile.

This component is crucial because it must be secure enough to withstand the force of the hammer strike while also ensuring that the cap can effectively channel the igniting flame into the firing chamber. The design and integrity of the nipple are vital for reliable performance and safety when using a percussion muzzleloader.