I know of a German 4x 20mm anti-aircraft gun. Here's the specs:
2cm Flakvierling 38
4 barrel 20mm AAA unit. Either towed, self-propelled, or static-type emplacement.
Caliber: 20x85.3mm
Effective ceiling: 3,500 ft
Max. horizontal range: 5,230 ft
Rate of fire [collective]: 800 rpm
Elevation: -10º to +100º
Muzzle velocity [HE]: 2,950 feet/sec
If you wanted something even worse than the M16 we've got now, this is it. Mount this quad 20mm unit on a half-track and you've got the ultimate in AAA defense. Think of an Fw-190 A8 that drives around and shoots at planes

Flakbait