The 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 14 hits in 10 seconds with a particle rifle. In 2153, Corporal Amanda Cole held such a record at level 4 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.