This unit is at the St. Petersburg Artillery Museum, Russia. It's a long-range chemical fire extinguisher based mainly on AK parts, made by Kalashnikov. The "gun" is not a converted firearm but made new from the massive supply of surplus gun parts. The magazine is actually the front grip. The idea is that the fire extinguisher's operation and ergonomics are familiar to Russian emergency responders.
And the Saiga on the side? A fireman's door breacher.