This phaser rifle, while not used as often as the 2370s-standard Type-3, is nonetheless a potent weapon, though more limited in utility.
The Compression phaser rifle takes its name from its unique ACB jacketing technology. While phaser pulses (ex. Defiant, standard Type-III) also use ACB jacketing to keep the blast of phaser energy coherent, the ACB jacket on a Compression rifle is constricting: it offers less space than the energy needs. As such, when the weapon is fired and heads toward the target, the energy begins dissolving the jacket, seeking release. Once the target is hit, a more explosive/radiant effect is achieved, along with much higher kinetic energy released. Time to target and power efficiency are lower than beam weapons, however.
This is useful for more heavily armed or durable opponents. When set to heavy stun, the explosive aspect of the blast is more efficient for distributing the paralyzing energy through the target. On higher levels, the enhanced kinetic energy and explosive aspect of the pulse delivers a much more punishing blow.
The Compression phaser rifle likewise constricts phaser beams, though the difference is less remarkable.
The downsides of these technologies are the increased power consumption, and their reliability--while tough, they have a reputation for not taking as much of a beating as other current Type-III models. The Compression phaser rifle is often used as heavy supporting fire for the lighter Type-III variants, and, along with the Type-III standard and Isomagnetic Disintegrators, plays a key role in a balanced Starfleet assault force.