The Micro Uzi is a weapon featured in the Thing-Thing series.
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 SMG ammo. To obtain the Dual Micro Uzis, you must beat the fifth level again with 225 kills.