Page "Rifling" Paragraph 21

The grooves are the spaces that are cut out, and the resulting ridges are called lands.

These lands and grooves can vary in number, depth, shape, direction of twist ( right or left ), and twist rate ( see below ).

The spin imparted by rifling significantly improves the stability of the projectile, improving both range and accuracy.

Typically rifling is a constant rate down the barrel, usually measured by the length of travel required to produce a single turn.

Occasionally firearms are encountered with a gain twist, where the rate of spin increases from chamber to muzzle.

While intentional gain twists are rare, due to manufacturing variance, a slight gain twist is in fact fairly common.

Since a reduction in twist rate is very detrimental to accuracy, gunsmiths who are machining a new barrel from a rifled blank will often measure the twist carefully so they may put the faster rate, no matter how minute the difference is, at the muzzle end ( see internal ballistics for more information on accuracy and bore characteristics ).