Disruptor Gun
This is the aliens step 3 or 4 weapon, moderately powerful, fairly accurate, and most of all infinite ammo. It packs far more firepower than the Megapol Laser Sniper Gun and Marsec M4000 Machine Gun, effectively outclassing either gun, and its abundance in the first 2 weeks or so makes it the staple weapon of X-com forces for the next 2-4 weeks. When soldiers have any armor other than Disrupter Armor, the Disrupter gun is also extremely deadly in close quarters combat, requiring only one or two hits to critically injure a soldier, and if flanked may allow an alien Anthropod or Skeletoid to kill a single soldier or unprepared squad. Aliens armed with the disrupter gun are the most dangerous aliens you will see in the first few weeks, with the only real rival in danger being multiworms and hyperworms in close quarters combat. Disrupter gun wielding aliens should be your first target in combat.
The Disrupter gun is best thought of as a sub-machine gun, for its rapid but not necessarily accurate fire. The closest human match to the Disrupter Gun would be the Plasma Gun.
Official Entry: "The Disruptor Gun is a remarkable piece of technology. It propels a beam of sub-atomic particles at immense speeds and quantity. The chamber which generates the energy is an inter-dimensional device which materializes energy from an alternative dimension. The power source, once initiated, is self-perpetuating and no ammunition or energy pods are required to sustain the fire power. Our replicators can reproduce this technology fairly accurately."