From UFStarfleet LCARS
The M-155A2 is the new standard Light Infantry weapon of the UFSMC. Introduced in the fall of 2375 as the M-155A1 (which is the same unit as the RP-155 carried by Starfleet Security), the A2 version adds luminescent lines on the dorsal side to serve as a low-light waist-aiming device for snap shots, an enhanced illuminated reticle system in the scope, and increased countermeasures against dampening. It is slightly heavier than the rifle it replaces (the M-110/Starfleet Type III), but is also more powerful. More importantly, it has a shoulder stock—the lack of which was the chief complaint about the M-110.
This rifle provides the necessary firepower for common direct-fire infantry situations. It possesses the full standard sixteen power levels of the Starfleet phaser rifle, with the addition of a pulse-burst automatic firing mode that conserves energy over the steady stream that is found on the M-970. It is reloaded from standard WC-2 power clips, of which three are normally carried in battle dress. The M-155A2 can have accessories such as laser-dot sights and beam scramblers fitted to it, in addition to the SPW-115A1 Weaponmount Grenade Launcher system. Like all phasers in the UFSMC inventory, its beam can be adjusted from a wide-field cone dispersion pattern to a "needle” pinpoint beam application.