The Krummlauf ("curved barrel") was a bent barrel attachment developed for the revolutionary Sturmgewehr 44 (StG 44), the world's first true assault rifle. The attachment came in several variants: a 30-degree curve for infantry use around corners and a 90-degree version designed for tank crews to fire at infantry climbing on their vehicles.
The device actually worked, using a curved steel barrel extension with holes drilled along the outer curve to vent gas and reduce pressure. A periscope sight was attached for aiming. However, the curved barrel had severe limitations: the bullets often fragmented upon exiting, turning into something between a bullet and shrapnel, and barrel life was extremely short—about 300 rounds for the 30-degree version and a mere 160 for the 90-degree variant.
About 10,000 units were produced. While impractical with the technology of the era, the concept was ahead of its time—modern armies continue to develop corner-shot weapons systems based on similar principles.