The reason chemical weapons are banned is because (they’re monstrous and) they’re useless. You can fire a chlorine shell and if the wind is juuust right, it’ll kill anyone within a few meters. You know what else will kill anyone within a few meters? A normal artillery shell.
Except, chlorine gas can be blocked by an airtight gas mask and a chemical suit. They cost less than $500 for complete immunity to the weapon. Good luck finding a $500 flak vest that’ll stop a mortar though. And meanwhile, if you want to press the attack and benefit from your chemical weapons, there’s one slight problem before you advance: there’s a bunch of chlorine gas in the way.
In other words, it’s an unreliable and inferior weapon that gets in the way of modern military doctrine. Although there are some good niches in shitty armies by dictators who are too paranoid of coups to give their junior officers any independence or proper kit. Like the Iraq army that the US army utterly steamrolled in 2005.
I think tanks would be their best bet - mounted machine guns and can carry lots of ammo, and if they’re surrounded and about to be overrrun then they can just drive away - through the zombies. It’s a tank, it has torque up the wazoo.
Actually, no, barbed wire would be their best bet - the blade doesn’t have to move if the zombies slice themselves apart on it.
In practice, just use both!