it's a 23mm, with twin barrels, hence the 23-2 in the name. Most soviet-era guns have a similar naming scheme like GSh-6-30 which is a six-barrel 30mm rotary cannon.

The ZU apparently stands for Zenitnaya Ustanovka, which apparently translates to anti-aircraft installation.

As for the damage it's a late 50's era weapon designed as a counter to helicopters and low-flying airplanes. The ammo varies from High-explosive frag to armor piercing rounds, and the polish also developed subcaliber APDS rounds but I'm not sure exactly how many of those you'd find in a place like this.

As far as the damage, the individual shots aren't the scariest by Anti-aircraft standards, but a 23mm frag shell is still going to cut down a person and the guys standing next to him, especially at a 2000 rpm cyclic rate (burst fire is necessary in practice otherwise the gun would melt.) Good for dealing with other soft targets like cars and trucks as well.

Sorry if that's a bit too much detail but I can't help it, I love old soviet garbage like this thing.