"Twist" is the term given to the distance within the barrel over which a rifled spiral completes one full revolution. Specific twists are better suited to optimizing the flight of bullets of differing weights or types within the same caliber.
As an example, consider the .223 caliber (5.56 mm) bullet popular in both military and civilian applications. A 1/12 twist (one revolution per twelve inches of barrel length) stabilizes 40 to 55-grain bullets well, but is unsuitable for heavier bullets like the 63-grain boat tail used in match target competition, or the 75-grain penetrator.
Conversely, a 1/7 twist, suited to stabilizing the heavier bullets well, is so rapid that it may cause potentially frangible bullets such as soft points to disintegrate while passing down the bore. This makes accuracy erratic, unpredictable, and unsatisfactory for bullets other than FMJ and JHP.
A 1/9 twist in a 20- to 24-inch barrel allows for essentially the full range of bullet weights, 40 to 75 grains, as well as for the use of potentially frangible bullets. Hence, it represents the ideal twist for this caliber because of its versatility. A 16-inch barrel in the 1/9 twist will accurately launch all bullet types in weights up to 69 grains.
Most manufactures attempt to offer the public a barrel twist that will accommodate a wide variety of popular bullet weights associated with the generally recognized uses of a particular caliber.