The particle rifle was a type of standard-issue carbine directed energy particle weapon. In the 22nd century, particle rifles were used by Zobral's clan on the Torothan homeworld and by United Earth's MACO personnel.
Inside Zobral's dwelling in the Cygniai Expanse, there were numerous particle rifles in a weapons rack which was usually hidden out of sight behind a tapestry. During a bombardment of their settlement, members of Zobral's clan armed themselves with the rifles. (ENT: "Desert Crossing")
The MACO particle rifle fired discharges of blue bolts of concentrated energy. The weapon could also be set to fire orange-red particle beams similar to those fired by a plasma rifle or a phase pistol. (ENT: "Chosen Realm") The particle rifle was capable of multiple power settings, ranging from stun, to kill, to a high-power shot capable of blasting through a Human torso. (ENT: "The Augments") Power levels could be changed by manipulating either of a pair of circular dials mounted on either side of the weapon. (ENT: "In a Mirror, Darkly") The weapon featured a tubular stock, vertical foregrip, and a multifunction scope that could be retracted into the body of the weapon. (ENT: "The Xindi")
While extremely powerful when set to kill, its stun setting appeared to be somewhat ineffective against species that were more physically resilient, such as enraged Vulcans, Xindi-Reptilians and the Augments. (ENT: "Impulse", "Rajiin", "Borderland") At their highest setting, MACO particle rifles were extremely powerful, capable of penetrating a humanoid target. In 2154, Captain Archer used a MACO particle rifle to kill the Augment Malik in this manner, using the rifle to literally bore a hole through Malik's torso. (ENT: "The Augments")
In a MACO weapons training simulation, a skilled user could score as many as fourteen hits in ten seconds with a particle rifle. In 2153, Corporal Amanda Cole held such a record at level four of the simulator, though she insisted she was just having a lucky day. (ENT: "Harbinger")
Particle rifles were in use as late as 2161, when a team of MACOs utilized the weapons during a mission to rescue Talla, the daughter of Shran and Jhamel, from a group of alien kidnappers on Rigel X. (ENT: "These Are the Voyages...")
Background information Edit
The designation and type of the MACO rifle was never identified on screen. The rifle is named as a "particle rifle" in Startrek.com. Based on effects it appeared to be the rifle-version of the MACO phase pistol and usually fired the same type of phase pulses. The MACO rifle was called an assault rifle,  an assault carbine,  a phase rifle  and an energy rifle  in the It's A Wrap! sale and auction.
Based on a scene in "In a Mirror Darkly", rotating the power level selector forward / away from the user appeared to increase the power level. There may also be a sniper variant of the rifle, though it is unclear whether or not the sniper version featured in "The Xindi" was a distinct weapon system or a standard MACO rifle with a retractable barrel or separate barrel attachment.
A short, tubular-shaped part of the rifle beneath the muzzle of the weapon that is very similar to a set of three objects the MACO's carry in a specialized pouch / carrier on their utility belts, suggesting the ability for the weapon to be reloaded. The MACO phase pistol and EM-33 pistol feature an identical object, also beneath the muzzle of the weapon, which may also be storage space for spare power cells for the rifle and pistol.