From Halopedia, the Halo wiki
The Machine Pistol is a human loadout weapon, exclusively seen in Halo: Spartan Assault's co-op Flood missions. It is an automatic weapon which fires very inaccurately and deals little damage, but has a fast rate of fire and a large ammo capacity to compensate.
Universe and lore
The machine pistol is a United Nations Space Command machine pistol of unknown make and model, which was in service during the post-Covenant War conflicts. At some point after December 5, 2557,[Note 1] the weapon was present in a TACSIM training session hosted by Roland aboard the UNSC Infinity. These used a simulation of a Forerunner location called X-40 which had been populated with Flood forms in order to prepare a group Spartans to fight the parasite, should the UNSC ever encounter it again.
The Machine Pistol is a light grey pistol with a short barrel, a foregrip, a tall sight at the front of the weapon and a long magazine protruding from the bottom of its rear grip. It fires very inaccurately, has a short range and deals little damage, however sports a very large ammo capacity and fires very fast. Like many other sidearms in Spartan Assault, the player's Spartan will dual-wield Machine Pistols when picked up.
At Halo: Spartan Assault's co-op loadout screen, if one of the two loadout options contains the Machine Pistol, the player can optionally equip it with one of three attachments, all of which increase its ammunition capacity by various degrees. These apply to all Machine Pistols in the match - not just the one the player spawns with. The upgrades will take effect for both players regardless of who bought it, and if both players purchase the upgrades, the increase in ammo will stack. This allows for a total ammunition increase of +90%. Upgrades will only apply for a single mission, and are single-use; they will need to be purchased again next time the player plays the same mission.
The three upgrades are as follows:
The Machine Pistol has no direct counterparts within the co-op Flood missions, as only a limited set of weapons appear in this mode. However, there are a number of other weapons in Halo: Spartan Assault and the Halo franchise in general which serve a similar role.
On the human, UNSC side, there is the SMG. Like the Machine Pistol, the SMG is an automatic sidearm, which deals very little damage per hit but has a fast rate of fire. The SMG appears in the Spartan Assault main campaign, where it fulfills a role very similar to that of the Machine Pistol, but is replaced by the Machine Pistols in its co-op missions. Like the Machine Pistols, it is dual wielded in Spartan Assault and is fully automatic, but it is significantly more accurate.
The Covenant uses a number of weapons similar in operation to the Machine Pistol. The Plasma Rifle and Brute Plasma Rifle are automatic weapons which fire plasma bolts which deal light damage, and are some what inaccurate. Furthermore, the Needler is an automatic Covenant sidearm which fires slow-moving pink needles that home in on the target, and can cause a "supercombine" explosion if enough projectiles connect with the target. Finally, the Spiker is an automatic weapon which fires superheated spikes which travel very quickly, but deals little damage to enemies. Of these four, only the Needler appears in Spartan Assault, being present in the campaign mode, but not co-op.
The Forerunner Prometheans have a similar weapon known as the Suppressor. It fires hardlight bolts which deal light damage to enemies. The weapon is incredibly inaccurate, but has a large magazine and fires very fast. Although it does not appear in Spartan Assault, the Suppressor is present in Spartan Strike, where it serves a gameplay role almost identical to that of the Machine Pistol, albeit with a much reduced ammunition capacity.
In general, the Machine Pistol is a weak weapon with a very specialised role overall; it excels at dealing with pod infectors, but is useful for little else. It can be found abundantly in all of the co-op missions, with at least two spawning on every level. Additionally, red combat forms will always be equipped with the weapon when they spawn. All co-op missions besides Co-op 1: Checkpoint feature the Machine Pistols in one of their loadout options.
The weapon bears the following stats in Halo: Spartan Assault:
The Machine Pistols are most effective against pod infectors. Ideally, they should be used from a distance, to allow the user to more easily escape if a swarm of enemies overruns them, which is liable to happen given the weapon's low damage. Unless there is no other choice, the weapon should not be used against combat forms, as it deals very little damage and forces the player to move in close and risk being meleed. The weapon is best used in combination with another, more powerful weapon; the Machine Pistols can be used to mop up swarms of infection forms without wasting ammunition for the other weapon, which the player can switch to in order to deal with more dangerous foes.
Despite not appearing in the game, the HUD icons for the machine pistol exist in the files of Halo: Spartan Strike, likely just a holdover from the first game. As with all HUD elements in Spartan Strike, these icons take on a slight green tinge when compared to Spartan Assault.
Curiously, the Machine Pistol takes on the appearance of the campaign's SMG when held by red combat forms, despite retaining the properties of the Machine Pistol. However, the combat forms will still drop a normal Machine Pistol upon death.
List of appearances