Borescope
A slender camera or optic that lets you inspect the inside of a barrel for fouling, erosion, and tool marks directly rather than guessing.
A borescope is a thin probe, either a fiber-optic eyepiece or a small digital camera on a flexible or rigid shaft, that slides into the bore and lights it from the inside so you can actually see the rifling. It turns barrel inspection from guesswork into direct observation, revealing how clean the bore really is, where machining marks sit, and how the steel is holding up. Many shooters consider one essential once they start chasing the last increments of accuracy.
Looking through a borescope is the only reliable way to judge several conditions that otherwise hide from view, including throat erosion at the leade, streaks of copper fouling in the grooves, and the hard carbon ring that can build at the case mouth. Seeing those problems directly tells you when cleaning is actually finished and when a barrel is approaching the end of its useful life.