The one aspect of this that is bothering me is the overuse of conventional weaponry as opposed to improvised techniques that would be far more effective. Such as mixing bleach and ammonia to create poisonous gas (if they're bulletproof from the outside, but still need to breathe air....)
Depending on which way the wind was blowing, a poison gas could be used at great distances and dissipate quickly.
...and what about common sense shit like BUILDING A MOAT? There's a reason moats were used extensively in the middle ages: they freaking worked. Or a page could be taken from Leiningen Versus the Ants, in which the protagonist uses a variety of moats and ditches.
By "moat", I don't necessarily mean a water moat, but a big ditch filled with jagged things would be highly effective. By leaving one or two paths of ingress/egress, it creates a bottleneck for the zombies. In all this time, nobody at Irwin or
the Colony considered this?