Type-31 needle rifle/Gameplay
From Halopedia, the Halo wiki
Compared with its counterpart, the M392 DMR, the needle rifle trades in damage per projectile for a faster firing rate, lower recoil/reticle bloom, and larger magazine size, and also has the ability to kill an unshielded enemy with three needles, which will detonate no matter where they hit. The needle rifle still retains the ability to kill an unshielded enemy with a headshot. Although most use it as a semi-automatic weapon, the needle rifle is in fact a fully-automatic weapon, and will fire continuously when the trigger is held. Its rate of fire is rather low, however, and it suffers from reticle bloom. Few players actually make use of its fully-automatic function.
The needle rifle is incredibly effective against Brutes. As most do not possess shields in Halo: Reach, they can be killed in three shots with a supercombined detonation. Enemies in Campaign and Firefight are extremely cautious when the needle rifle is used against them, regarding its projectiles as if they were explosives. Grunts dodge them the same way they dodge plasma grenades, Brutes will frequently dodge or use Armor Lock when shot at, and Elites will evade constantly even if their shields are at full strength. This quality makes it a somewhat ineffective long-ranged weapon unless one finds oneself able to fire into a crowd of enemies. Noble Team members will also react to enemies using needle rifles by Armor Locking, despite the fact they are invulnerable.
The needle rifle will take out a Spartan's shield in Multiplayer with 6 rounds, 2 more than the 4 rounds required by the DMR, but does "bonus damage for headshots." Three needles in an unshielded target will cause a supercombine detonation, similar to the needler, killing the victim. Thus, in most SWAT game types, this weapon is not present due to the fact that it can kill an enemy quickly without the requirement of a headshot.