Needle cannon

Introduction
The Heavy Needler is only seen in Halo 2. It is first seen on the level Sacred Icon when Tartarus explains to you how to destroy an Enforcer. If you stay alive long enough, the Enforcer will start to shoot at you with its Heavy Needlers, which fire crimson needles that are larger and more powerful than the weaker purple variety which the Covenant use. These needles don't home in on a target or do not explode on contact, but still pack quite a punch and can take down the Arbiter (you) after a couple of direct hits. This weapon is particularly useful against the hordes of Flood that the Sentinels face and was most likely developed specifically for combat use against an infected lifeform. The Enforcer will attack you with it on the levels Sacred Icon and Quarantine Zone.

Use
The Enforcer will only engage targets with the Heavy Needler that are few and far between, such as the Arbiter, and very small groups of Flood. The Enforcer is not known to otherwise engage targeted hostiles with this weapon.

In combat, it acts very similarly to the Brute Spiker, which the same damage and same method of inflicting it. The needler, in contrast, explodes upon contact, though the Needler is more close in terms of size/colour.

Speculation
The Heavy Needler fires and uses rounds somewhat similar to the Needler. It is possible that Needlers were reverse engineered from the Enforcers' weapons. Bungie did plan for another Sentinel weapon to be usable by the player referred to as the "Heavy Needler", "Sentinel Needler", "Red Needler", and/or "Red Spiker". For reasons left only to Bungie, this weapon was not in the game, but the Enforcer's weapon is most likely the same gun. It is entirely possible that Bungie had already been testing them in gameplay on sentinels, because there is a glitch that gave one Sentinel, known simply as the Needler Sentinel, a normal needler.

Trivia

 * The Heavy Needler can't be used by the player.