List of British Ordnance Terms - Windage

Windage

"Windage" as applied to British muzzle-loading ordnance referred to the difference between a gun's bore and the projectile's diameter, typically 0.1 - 0.2 inch. This gap was necessary to allow the projectile to be rammed down the length of the barrel on loading. The word windage was also used for the amount of propellant gas that escaped around the loosely-fitting projectile on firing, and hence failed to contribute to accelerating the projectile. Up to half of the gas was lost in this way in old smoothbore artillery. From 1859, Armstrong rifled guns used a deformable lead coating on the projectile to minimise windage and simultaneously to engage the rifling. The elimination of windage necessitated a new design of timed fuze, because the burning propellant gas escaping past the head of the shell had been used to ignite the gunpowder timer train in the fuze in the shell nose. The new fuses used the shock of firing to ignite the timer. When Britain reverted to muzzle-loaders in the late 1860s, projectiles were rotated by studs protruding from the shell body engaging in deep rifling grooves in the barrel, but the windage caused excessive barrel wear. From 1878, after several years of unsuccessful trials, a fairly effective system of concave copper discs called gas checks was introduced between the charge and projectile; they expanded on firing and sealed the bore. The gas checks were soon incorporated into the projectile itself and became the driving bands still in use today.