Heckler & Koch P2A1
It can fire either 26.5 mm or 25 mm flares, or smoke cartridges. Adapters are available to use commercial 12 gauge flares available at sporting goods stores.
The body of the pistol is mostly polymer composite, with the barrel and breech being made of steel. Very rugged and simply designed, the P2A1 has a life expectancy of about 1500 shots of full power flares. Past that point, barrel erosion makes the weapon unsafe.
The P2A1 is in use in the German army, called "SigPi" (short for "Signalpistole" or signal handgun).