Lots of people have different opinions of what it was.
Heres a few ideas I found.
ZSU 14.5mm antiaircraft gun
Modified .50 caliber machine gun
Vladimirov KPV 14.5mm machine gun
Polish ZSU-33 Electric 14.5mm 7.62 caliber
Someone even said it was a 152mm cannon ;D
On the DVD commentry the director said it was a made up gun though (If I remember rightly)