The Micro Uzi is a weapon featured throughout the Thing-Thing series.
The Micro Uzi is a compact variant of the world-famous Israeli submachine gun designed by Major Uziel Gal. It is chambered in 9mm Parabellum, and fires at a rate of 1700 rounds per minute, extremely fast. It is also small enough so that two of these weapons could be wielded at once. While the high rate of fire makes the weapon eat up ammo, it's useful for mowing down enemies.
In Thing-Thing 1, the Micro Uzi can only be unlocked by activating the cheat code in the main menu. The Micro Uzi is depicted with having its barrel rise when you hold down the trigger long enough, so it's recommended to fire in short bursts to avoid this. In addition, its rate of fire is slower than in real life, but considering that Thing-Thing 1 is essentially a test engine for the later games in the series, this wouldn't be surprising.
The weapon returns in a dual-wield form in Thing-Thing 3, with a rate of fire rivaling that of the M249 SAW, and can mow down enemy soldiers just as effectively. However, that rate of fire can really use up your SMG ammo, so save up on it as much as possible. To obtain the Dual Micro Uzis, you must beat the fifth level again with 225 kills. A daunting task, but this weapon's firepower is well worth it.