When John Browning designed his Model 1895 machine gun with it's rotary-lever gas operation system, Hiram Maxim filed suit claiming patent infringement. Maxim had filed quite broad patents covering gas pistons operation, but specifically in a linear format. Browning and Colt (who had the license to manufacture the Model 1895 machine gun) countered that the swinging lever was a different system, and thus not covered by Maxim's patents. More to the point, they claimed that the gun would work without using a gas piston at all - and built this experimental model using a gas trap or muzzle cap system instead to prove the point.

Ultimately, the genesis of the fight was moot (the Maxim did not run well in 6mm Lee Navy, and would not have won a US Navy contract regardless of the Colt/Browning gun), and the court ultimately decided in favor of Colt and Browning. But this gun remains from the incident...