From Halopedia, the Halo wiki
The SRS99 series is designed to be used at medium to long ranges and is equipped with an electronic scope for precisely this purpose. It is tremendously useful in its designated role, considering its extraordinary range, which surpasses any other weapon of comparable size, and its incredible stopping power even against body armor and Covenant shielding system.
By far, one of its most useful applications is taking out higher-ranking targets from long range to destabilize their squad's discipline and morale. A headshot will kill any Covenant infantry except a high-ranking Elite or Brute. Hunters fall from two shots to their unarmored torso or neck. This does not account for The Flood, however, due to their morphology. The Flood can, however, be rendered harmless by removing their limbs or by placing a round into the embedded infection form. In multiplayer, the SRS99 series can be highly effective at close range in the hands of a skilled operator, as the highly damaging round is just as powerful at close range; a single round followed by a melee strike will kill a hostile almost instantly.
The rifle's projectile has a slight travel time at great distances, and requires practice to use effectively at range. When used in close quarters, its precision nature, slow reload time, small magazine capacity, heavy recoil, and slow rate-of-fire leave precious little room for error. A small combat load and the relative scarcity of ammunition available for the weapon also make it bad practice to expend ammunition on smaller, less threatening combatants, such as Grunts. The SRS99 series is also largely ineffective against most Flood forms, due to their body organization; damage done by the SRS99 series/Gameplay is minimal to the Flood forms due to their biological constitution and would only cause the Flood form to flinch for a moment. It is often recommended for operators to simply switch to other weapons should they encounter a Flood form.
As in all previous games, rounds fired by the SRS99 series leave a trail that could be traced back to the sniper's location. When using the scope, the glare from the reflection will be visible to the enemy and may alert them to your presence. The rifle is ineffective against even light-skinned and unarmored vehicles unless a shot can be taken on the vehicle's occupants, except for Wraiths.