The rifle (in the shooting sport) is single - loaded in _________-millimetre calibre (the inner diameter of a gun barrel), which is used across all events.

Explanation

5.6. Olympic shooting uses rifle, pistol, and shotgun events at 10m, 25m, and 50m. Rifles (5.6mm) and pistols (4.5mm air / 5.6mm rapid fire) are mostly single-loaded, while shotguns are 12-gauge (18.5mm), where gauge means how many lead balls from a pound fit the barrel.