A phaser rifle (often referred to as a Type-3 Phaser) is a rifle variation of the standard phaser used by the Federation.
24th century phaser rifles have sixteen power settings, fully-autonomous recharge capability, multiple-target acquisition, and gyro-stabilization. Training for Type 3 Phasers is only available on Starbases.  They are slightly less powerful than Cardassian phase-disruptor rifles, and are considered by some to be less effective as a field weapon due to their complexity, despite being more powerful than the standard phaser. (DS9: "Return to Grace") Compared to the Type 2, Mid 2360s Federation Type 3 Phasers have four prefire chambers and 50% greater power reserves. 
Phaser rifles were not standard equipment aboard Starfleet vessels prior to the early 2370s. Spock had to make a special request for one to be brought down to Delta Vega in 2265. (TOS: "Where No Man Has Gone Before") Typically, rifles were requested when firepower heavier than that of the type 2 phaser was required.
Facsimile phaser rifles were created by the Romulan Star Empire in 2367 and given to rebels on the Klingon planet Krios, in an attempt to destabilize relationships between the Federation and the Klingon Empire. Both the facsimile and Federation rifles had an output of 1.05 MW. The only physical difference to the standard Federation rifle was an emission crystal efficiency of 94.1%. This was eight percent above the standard model. Another difference was that the initial output spike of the phaser beam was inverted, a sign that the power cell had been charged with a forced pulse in the terahertz range – a method used in Romulan disruptors. (TNG: "The Mind's Eye")
The 2360s version of the rifle could be modified with the use of a tracking light mounted on the central body, as well as a shoulder strap designed for ease of carrying. (Star Trek: First Contact) A phaser rifle could be set to fire an expanding energy pulse. The pulse could be set low enough to avoid damaging equipment, but high enough to affect changelings. It also heated up the air it was fired through. (DS9: "The Adversary")
The compression phaser rifle was a new phaser design used aboard Intrepid-class starships such as the USS Voyager. (VOY: "Caretaker")
More advanced phaser rifles capable of firing phaser bolts as well as the standard beam were used in the 2370s. These rifles had a pistol grip in back and either an "underbarrel grip" or a second vertical grip underneath the barrel (similar to conventional firearms of times past), and were capable of being modified with various types of scope, barrel and power cell. (Star Trek: First Contact; Star Trek Nemesis) When Worf's rifle ran out of power on Ba'ku, he improvised when he swung his at a Son'a drone when it was firing transporter tags at Baku villagers and Starfleet crew. (Star Trek: Insurrection)
Another rifle variation was in service at the same time, and used during zero-gravity EVA-operations in the Borg incident of 2373/2063. This phaser rifle variant was a return to the two-handed grip seen earlier. This variant was able to be magnetized against a surface, further proving its usefulness in zero-gravity environments. (Star Trek: First Contact)
The new 2370s rifles were differentiated as Mark I, Mark II and Mark III in the It's A Wrap! sale and auction. Additionally, the new rifles were identified as compression phaser rifles in Star Trek Encyclopedia (3rd ed., p.356) and in the apocryphal Star Trek: Voyager - Elite Force and Star Trek: Elite Force II video games.
For settings and effects details, see entry for Federation Type 2 Phaser
- ↑ 1.0 1.1 1.2 1.3 Memory Alpha Article on the Type 3 Phaser
- ↑ Sternbach, Rich and Michael Okuda. 1991. Page 137.
- ↑ Sternbach, Rich and Michael Okuda. 1991. Page 135.
- ↑ Sternbach, Rich and Michael Okuda. 1991. Page 134.
- Sternbach, Rich and Michael Okuda. Star Trek The Next Generation Technical Manual. Boxtree Limited. 1991. ISBN 1-85283-340-8.