The phaser rifle is a larger, two-handed form of the phaser that delivers more power and range than a pistol, used by Starfleet security teams and landing parties.
Captain Kirk carried a phaser rifle when he confronted his former friend Gary Mitchell on Delta Vega, a powerful weapon chosen because Mitchell's growing psychic abilities made him so dangerous. The rifle was rarely seen in the original era, but the concept returned in force in the 24th century.
By the time of the Enterprise-D and later ships, phaser rifles were standard equipment for security details, away teams facing heavy resistance, and boarding actions. Crews fighting the Borg relied on them heavily, although the drones' ability to adapt to repeated phaser fire meant frequencies had to be changed constantly.