The owner of the video says he was driving his wife to the airport in Moscow when he reduced speed after seeing a 40km/h (25mph) sign on the road. Apparently, the driver behind him thought he was trying to teach him a lesson and didn't appreciate it. Naturally as it's in Russia, it turned into a road rage incident.
Btw, he didn't shoot his gun, he used it to hit the other guy in the face.
Even weirder than your average 'Meanwhile, in Russia' video...