A curved-barrel firearm design, intended for urban combat. Originally developed by the Germans in WWII, and then experimented with by the Russians. Available in 30°, 45°, 60°, and 90° bends.
The first example was the Krummlauf, a barrel attachment for the German Sturmgewehr 44:
Then the Soviets experimented with this concept on a PPSh-41:
The curved barrel attachments had extremely short lifespans, around 150-300 rounds, depending on the angle of the barrel. Bullets would also occasionally shatter in the barrel, producing a dangerous shotgun-like effect.
Forgotten Weapons covering the Krummlauf barrel:
This concept is similar to the WWI-era trench warfare periscope rifle, which was essentially a rifle modified to be sighted with a periscope.
A 1915 WWI photo of a periscope rifle and its inventor:
The modern era equivalent is the CornerShot, invented by Amos Golan of the Israeli Defense Forces for hostage rescue, although now available in the civilian market. Interestingly, it's more similar to the periscope-style WWI version. The barrel doesn't curve; various handguns can be inserted into the CornerShot, which can then be angled. Targets can also be viewed around a corner via a video camera attached to the CornerShot.