No matter in what game, the M41 Rocket Launcher is always the best choice for passengers on any vehicle looking to wield heavy firepower. The M6 Spartan Laser and Plasma Launcher take too long to charge, while the Fuel Rod Gunprojectiles are too slow. However, the Rocket Launcher is still very slow to fire, and missing while on the move is remarkably easy, so it is sometimes advisable to take a less powerful "spray and pray" weapon instead.
A Gauss Hog with a Rocket Launcher-wielding passenger is an absolutely lethal vehicle. Stay mobile, letting your gunner and passanger take potshots at the enemy before retreating out of range of any return fire.
In Halo 2 and Halo: Reach, the Rocket Launcher has the ability to lock on to enemy vehicles, and this plays a vital role when facing fast moving vehicles such as Banshees or Ghosts. In Reach, though the rockets will only lock onto aircraft when used by the player, it will also lock onto ground vehicles and even infantry when used by an AI ally.
In Halo 3: ODST, AI characters' accuracy seems to have improved for gameplay balance. Give them the Rocket Launcher to strengthen one's allies and their firepower.
In Halo 3 and Halo 3: ODST, the M41 Rocket Launcher loses its tracking ability, so it is not recommended to fight aerial vehicles. Use it against enemy vehicles, provided that they are close, and large groups of infantry. It is also, unsurprisingly, devastating against Hunters.
In Halo: Reachcampaign level The Pillar of Autumn, it is better to save the M41 Rocket Launcher until one reaches the battle under the UNSC Commonwealth in the ship breaking yards. While the Sangheili will dodge the projectiles, predict where they would move to next and fire at that position. Even if the rocket doesn't hit them, use splash damage to lower their energy shielding. Once your almost out of ammo, give the weapon to an AI.
In Halo: Reach, the M41 Rocket Launcher can be used to get the "Lucky Me" achievement with relative ease. The best mission to do this on is Exodus, just after acquiring the jetpack to take the executive landing pad with the Bullfrogs. Grab the launcher wielded by the ODST just after you get out of the elevator, jetpack across to the other side and fire while in mid-air against the many Covenant troops milling around below you.
In Halo 2, the M41 Rocket Launcher has a long range for locking on enemy vehicles. Whenever an opposing player commandeers a Banshee, use the Rocket Launcher to take it down.
This also works in Halo: Reach (as the lock-on ability was removed in Halo 3). However, be wary that a good Banshee pilot can, and will, throw their aircraft around the sky in an attempt to break the lock or avoid the rocket. Make sure that you have a solid lock and (if possible) they are engaged in other combat with your team before firing.
Falcons in Halo: Reach are much less troubling (as they can't preform evasive maneuvers like a Banshee can), but have their own problem; if timed correctly by the pilot, a rocket can and will fly straight through the open passenger bay in the center of the Falcon and keep on going. This is not a common occurrence however,and applying the same tactic as above should ensure that the Falcon will fall.
Never aim directly at an enemy player on foot with a rocket launcher. The small, fleet tendencies of an enemy soldier makes it nearly impossible to score a direct hit. Instead, aim for their feet, and perhaps a little bit a head of where they are. Players near the rocket's splash radius can be killed, and if they aren't killed, keep a secondary weapon (preferably a precision one) to finish them off with a quick headshot.
Despite the obvious intended use of a rocket launcher (that being anti-vehicle combat), the rocket launcher is not actually as effective against vehicles at it would at first seem. The sheer maneuverability of most light vehicles means that they can easily avoid a slow-moving rockets at anything other than near-point blank range, and heavy vehicles can shrug off entire magazines worth of rocket hits (assuming you get close enough to accurately fire without being killed by said vehicles). Save your ammunition for infantry, or for use against stationary vehicles (such as a Warthog awaiting an objective carrier, or any vehicle hit with the EMP pulse of a Plasma Pistol).
Halo 5 features multiple variants of the SPNKr, ranging from the SPNKr EM, featuring four rockets per magazine instead of two and the ability to lock onto both ground AND air vehicles, the SPNKr EX, which fires hardlight explosive rounds, and the SPNKr Prime, which not only boosts speed and jump height but fires insanely fast rockets.