Caliber

Good performance requires a concentric, straight bore that accurately centers the projectile within the barrel, in preference to a "tight" fit which can be achieved even with off-center, crooked bores that cause excessive friction, fouling and an out-of-balance, wobbling projectile in flight.

[citation needed] While modern firearms are generally referred to by the name of the cartridge the gun is chambered for, they are still categorized together based on bore diameter.

However, there can be significant differences in nominal bullet and bore dimensions, and all cartridges so "categorized" are not automatically identical in actual caliber.

[3] One of the early established cartridge arms was the Spencer repeating rifle, which Union forces used in the American Civil War.

Variations on these methods persist today, with new cartridges such as the 204 Ruger and 17 HMR (Hornady Magnum Rimfire).

The gauge of a shotgun refers to how many lead spheres, each with a diameter equal to that of the bore, that amounts to one pound (454 g (1.0 lb)) in weight.

The term caliber is used as a measure of length of artillery barrels from muzzle to breech, expressed as a multiple of the bore diameter.

The British Royal Navy insisted on 50-cal guns on ships as it would allow 1,900 to 2,700 lb (860 to 1,220 kg) shells to travel at an initial velocity of up to 1,800 mph (2,897 km/h) to a distance of 26 mi (42 km).

From about the mid-17th until the mid-19th century, the measurement of the bore of large gunpowder weapons was usually expressed as the weight of its iron shot in pounds.

In practice, though, significant variation occurred in the actual mass of the projectile for a given nominal shot weight.

Complicating matters further, muzzle-loaded weapons require a significant gap between the sides of the tube bore and the surface of the shot.

This is necessary so the projectile may be inserted from the mouth to the base of the tube and seated securely adjacent the propellant charge with relative ease.

The gap, called windage, increases the size of the bore with respect to the diameter of the shot somewhere between 10% and 20% depending upon the year the tube was cast and the foundry responsible.

The relationship between bore diameter and projectile weight was severed following the widespread adoption of rifled weapons during the latter part of the 19th century.