I was thinking about this very topic about a week ago. This answer is in relation to the Twilight war not so much a zombie outbreak.
There was probably a two maybe even three year period of full tilt production by every ammunition manufacturer worldwide for producing military calibres and production of hunting and civilian use ammo would have ceased. So 5.56N or 7.62x39 would still be available even years after the nukes fly. More obscure calibres with no military use like .38 Special or .44 Magnum would be rare in comparison and only available in small lots.
I would imagine Neutral nations would cash in on ammo sales after the nukes fly due to the worsening global logistic system.
I found this article:
http://www.almc.army.mil/alog/issues...arms_ammo.html