This cannon might be tough if you don't have a super chaingun. It shoots with a blue electro-ray which explode on impact, so it can seriously damage your health.
Large Cannon
Many of these cannons will try to shoot you in the shooting range while practising their aim on humans' portraits. You can't destroy them.
Turret
Not too tough. Several of them guards asylum level.
Spaceship Gun
Attached to the Spaceship and the Battlecruisers. It shoots with a blue electro-ray.
Ground Cannon
Operated by Grunts, it's very tough cannon. It shoots with blue electro-ray which explode on impact. It might be hard to destroy it with just a chaingun. You must aim at the small gap in sniper mode, because Grunt's head is exposed to sniper fire. One shot in the head should be enough to destroy this cannon.
Floating Cannon
This cannon floats above the fan.
Cannon Tower
There is no difference between this and the previous cannons. It is also very dangerous.
Bulletproof Cannon
Operated by Grunt Commanders, this floating cannon fires a special bouncing grenades. Chaingun is useless against it, but your grenades should do the trick.
Pyke's Spinning Wheel
Spinning machine with four dangerous bullets, powered by stationary bicycle, on which sits a boss in a straitjacket. Its glass is bulletproof, so chaingun is useless.