This was the Soviet answer to the US Army's Project Salvo from the 1950s/1960s. You might get some more info by looking for Pribor with the term 'meroka' included in the search (meroka is a German term - MeRoKa from Mehrrohrkanone, meaning multi-barrelled gun).
It might also help to look for it with reference to the Nordenfelt Gun.
There's been lots of speculation about the recoil. Despite the weapons overall weight, the recoil from 3 barrels firing at the same time cannot possibly increase your chance to hit and the entire purpose of the weapon was to increase the hit chance!
Here's one thread with a few more images.
http://www.defensivecarry.com/forum/...ult-rifle.html
P.S. I believe the second model of the Pribor had forward ejection of the cases, I only remembered that after having another look at the photos!