Ammo stockpiles

[aspqrz]

An email from a correspondent of mine regarding a particularly silly apocalypse proposal thread on Reddit got me thinking about something that had sort of been kicking around in the back of my head for a long while ...

Assuming something like a 'Walking Dead' outbreak which largely overwhelms society before it can work out a solution, how much ammo is actually on hand at any given time (I'm mainly talking small arms ammo, not large caliber stuff) in military stockpiles?

I mean, a lot of Zombie or similar post- apocalypse fiction (not all of it, but a heck of a lot) seems to assume an almost unlimited supply ...

Remembering back to the first episode of TWD, when Rick wakes up and travels through the town seeing the overrun defensive positions etc. Or the last episode of the first season when they get to the CDC and find the army positions outside overrun because they ran out of ammo ...

Obviously, at some point, the ability to manufacture ammo breaks down in the face of the disaster ... but how much ammo would be on hand?

The best figure I have been able to come up with is the assumption that NATO (mainly the US) may have had 90 days stockpiles, and more (maybe another 90 days) in the Continental US ... but how long would it last against an enemy who can only be put down permanently by a headshot?

Anyone have any idea what sort of stocks of small arms ammo were held? Are now held?

(Yes, I know Zombie apocalypses are pretty close to being the least credible apocalypse scenarios out there ... but the question of how much ammo there is is ... interesting ...)

Phil