The Einhorn is a fictional sniper rifle introduced to us in Jurassic Park 3, which, according to the dialogue, fires a 20 millimeter diameter projectile. The real world weapon seen in the film, however, is actually the Barrett M82A2, which is bullpup sniper rifle that fires the .50 Browning Machine Gun cartridge. It is a variant of the better known M82A1, and no longer in production.
The Einhorn was seen only in the early scenes of JP3 in Cooper’s possession. In the scene where the rifle was first revealed, Cooper was using armor piercing incendiary rounds, which had a very destructive effect on the target. Cooper was also seen carrying the rifle into the jungle on Isla Sorna, until what is presumed to be an off-screen encounter with the Spinosaurus, during which shots were heard. After the first shots were fired, Nash and Udesky were clearly seen boarding the plane when a second, louder, burst of shots could be heard. It was certainly Cooper firing those last shots, but whether it was the Einhorn that was fired can only be inferred, as the action takes place off camera. The Einhorn was never seen again after this point, as Cooper emerged from the jungle without it.