The beam rifle

A Particle Beam Rifle is a battery-powered directed-energy weapon that fires ionized particles at its targets, instead of accelerating small projectiles. The weapon is nearly silent, negligible muzzle signature, weighs considerably less than its UNSC analog and inflicts wounds akin to the 14.5mm SRS 99 AM — and is considered by all assessments to be a force multiplication. The use of Particle Acceleration offers two distinct advantages, the first is its damaging properties over the UNSC Sniper Rifle, as the ionized particles can pass through material easily, making it effective against targets behind cover. The Type-50 SRS also excels at target penetration, maximizing its lethality, as the beam will always pass through a target completely. Although the size of the wounds inflicted on a target are small, the damage done is similar to most Directed Energy Weapons used by the Covenant.
The Type-50 SRS is unusually shaped, and takes design cues from the Covenant Plasma Rifle in several respects - the barrel of the weapon is very long with its scope located on the top, and appears to slide into the weapon for storage. The scope on the particle beam rifle has two levels of zoom for greater accuracy, like its UNSC counterpart it is capable of 5 and 10 times magnification. The round section in front of the hand guard and trigger area of the weapon appears to contain the Type-50s "ammunition", after the weapon has completed its firing sequence, the section rotates presumably to "chamber" another "round". Human military experts do not understand how exactly the weapon works or how it is reloaded or recharged. Throughout the Human-Covenant War the particle beam rifle is not encountered often, and it is not until the final years of the war that it becomes much more common. Whether it is indicative of a widespread change in battlefield doctrine or a local reorganization of unit structure is unknown.I have got info from halopedia.com