a) Cost, obviously.

b) What makes you think a 20mm cannon can stop the God tier zombie? That thing is between 5 and 6 times as tall as an average person and even thicker by ratio. Just by simple division that means your gauge is equivalent to 3.5-4mm in relation to an average human sized zombie, effectively you're trying to kill this thing with the gun attached in the picture.

Furthermore, 5-6 times the size of a human is over 3 times the size of an average elephant, which is what a 20mm gun is generally designed to have stopping power against. Couple that with the fact that it takes even more comparative stopping power to take out a zombie, and your base is looking pretty roflstomped.

c) No spare parts

d) You clearly have no idea how a septic tank works

e) I have a couple friends in HVAC, it's 4 in the morning here so I won't bug them now, buy I'll get back to you on whether or not your HVAC system is up to snuff

6/10, blows the competition out of the water in terms of size and thought, but considering that you have clearly considered this possibility before, I expect you to have put more thought into the details.

And your forever stuck there if your helicopter runs out of gas.

But you forgot something important. There's always one in the bathroom.

One elevator shaft that goes through every single solid steel protection plate? Now that's just asking for trouble...

...damn
have my internets. Just take them.

Why does the armory have bullet-proof vests for an onslaught of zombies?