Mechanics
Bore
The hollow interior of a barrel through which the bullet travels; its diameter defines caliber or gauge.
The bore is simply the inside of the barrel — the tube the projectile travels down. Its diameter sets the caliber (for rifles and handguns) or gauge (for shotguns). A rifled bore has lands and grooves to spin the bullet; a smoothbore (most shotguns) has none. Bore condition — clean, pitted, worn — directly affects accuracy.
You'll hear bore-related terms often: 'bore axis' is the height of the barrel's centerline above your grip (a low bore axis reduces muzzle flip), and 'cleaning the bore' is routine maintenance with a rod and brush. Keeping the bore clean and properly lubricated preserves accuracy and reliability.