Soldiers equip weapons based on their class, which can be seen in the Barracks or while selecting a squad for a mission. Alien species always make use of the same weapon(s) with the exception of the Muton (see Alien Weaponry below). While weapons can be captured from aliens which have been stunned by the Arc Thrower or had the difficulty turned down to a point where their health value becomes 0, they cannot be used until the associated research project has been completed. Newly researched weapons become available for production in Engineering whereas conventional weapons (available by default) are already stocked in unlimited quantities.
Soldiers carry a primary weapon, secondary weapon, and an item (two with Tactical Rigging in XCOM: Enemy Within). Primary weapons are a soldier's main source of firepower in combat; they include rifles, sniper rifles, shotguns, and LMGs. Secondary weapons vary; they include pistols, rocket launchers, and a wide range of tactical subsystems available to MEC Troopers that are installed directly to a MECEW. A soldier's item slot can hold a variety of grenades, a S.C.O.P.E. to improve equipped weapons, or an Arc Thrower to capture live aliens, among other useful equipment.
On occasion a class ability (such as the Sniper's Headshot or a Heavy's Mayhem) is related to the weapon's technology level. These are, in order:
MEC weapons have the lowest ammo capacity in the game, permitting only two shots per reload. The Ammo Conservation Foundry upgrade allows two additional shots, and the Expanded Storageability one. These stack for a potential five shots per reload.
Mutons use Light Plasma Rifles when first encountered (in May) before switching to Plasma Rifles for the remainder of the game. This change occurs in June on Classic/Impossible difficulty, or in July on Easy/Normal difficulty.
Cyberdiscs, Heavy Floaters, and Muton Elites can throw grenades farther due to their Bombard ability.
In addition, many aliens employ specialized weapons or offensive capabilities that are unavailable to XCOM and unique to their species:
Weapons are subject to Aim bonuses and/or penalties, based on the weapon type and distance from the target.
Weapons are zeroed (no Aim bonus or penalty) at a range of 10 tiles. The Aim bonuses and penalties above are based on straight-line distances and cardinal unit positioning. Angled positioning with a Shotgun or Sniper Rifle can result in an Aim bonus/penalty corresponding to values within ±1 tile of the straight-line distance (calculated using Pythagorean theorem).
In XCOM: Enemy Within, critical hit damage may vary from what is listed in a weapon's Tactical Info due to a calculation bug. They may inflict one point less than the indicated minimum, and/or be limited to one point less than the indicated maximum.
EXALT does not utilize shotgun type weapons, nor do they use pistols. Their gear also caps at laser-based weaponry in terms of advancement, therefore no plasma-based EXALT armaments can be seen on the field.
Aliens utilize neither shotgun type weapons nor sniper rifle type weapons, although they have some more unique weaponry.
A soldier can shoot to any enemy less than 18 tiles away, unless there is some obstacle between him and enemy.