Gain Twist
Also: Progressive Twist
Rifling whose twist rate accelerates from chamber to muzzle, engaging the bullet gently at first and spinning it fully by the time it exits.
In a gain-twist barrel the rifling starts with a slow rate of turn near the chamber and steadily tightens toward the muzzle, so the bullet leaves with a faster spin than where it began. The shallow initial angle eases the projectile into the rifling, reducing the abrupt engraving force that a constant twist applies the instant the bullet starts to move. By the muzzle the twist has reached its final, faster value.
The intended payoff is full rotational stability factor at exit combined with gentler engagement that can lessen jacket stress. Cutting a gain twist is more demanding than cutting a fixed rate, so the feature appears mostly on premium and purpose-built barrels. Like any spinning bullet, one launched from a gain-twist bore still produces spin drift downrange, which the shooter accounts for at long distance.