From Halopedia, the Halo wiki
The M57 Pilum Assault Weapon (M57 PAW), also known as the Rocket Launcher, is a shoulder-fired heavy weapon issued to personnel of the UNSC Army, UNSC Marine Corps, and the Spartan branch around October 2558. It performs very similarly to the M41 SPNKR, its predecessor.
Despite performing much the same as the M41 rocket launcher, the M57 rocket launcher is cheaper and substantially lighter. The M57 rocket launcher is also significantly different in design philosophy, featuring a single, reusable, fixed tube and detachable box magazines rather than dual, rotating, pre-loaded, disposable tubes.
The weapon is fed by a two-round detachable box magazine on the upper left side, forward of the user. The magazine retracts into the weapon when chambering each round, similar to the Pez'tk-pattern fuel rod gun. A 2× magnification scope folds out from a module on the left side when activated. Its missiles are able to lock onto aircraft if the operator uses smart scope. Standard warheads used by the rocket launcher are 50mm high-explosive multi-purpose, but specialist rounds are also available, including those with thermobaric fillers or dumb AI-driven "brilliant" guidance systems.
The M57 Pilum is featured as a usable weapon called the "Rocket Launcher", in Halo 5: Guardians. It is a power weapon, which fires slow-moving rockets that deal an incredible amount of damage upon detonation, with a wide area of effect. The Rocket Launcher is one of the most powerful weapons in the game, and to compensate, it has a small magazine size, slow reload speed and ammo is generally very rare.
Non-canon and dubious canon appearances
M57 Pilums are used by UNSC personnel in battles against the Covenant.
Halo 5: Guardians REQ cards
List of appearances