The flamethrower

The NA4 Flamethrower is a type of flammable defoliant projector used by the UNSC Marine Corps that was used prominently in battles during at least2531. Similar in function and use to its later successor, the M7057 Defoliant Projector, the NA4 was much larger and bulkier and, rather than attaching its fuel tank to the frame of the projector, the tank was carried on the back of the user, allowing greater capacity. In addition, the weapon used a range of flammable defoliants, to compensate for the variety of atmospheric and gravitational conditions it could be used in. It is unknown whether the weapon was phased out of service, or whether it continued to serve alongside the smaller and more portable M7057. ^[1] During its service time, it was mainly used by Hellbringers Marines.

As a flamethrower, its primary function was to remove entrenched enemies. As such, the users of these weapons have a high attrition rate, despite the devastating damage they can inflict, due to the exposed fuel tanks that were a huge weak spot. Also, the NA4 is known to be compatible with the M301 40mm Grenade Launcher attachment used to launch flashbangs.I have got this info from halopedia.com