Supply of ammo that could be monthly (8)
I believe the answer is:
magazine
I believe this is a double definition.
'supply of ammo' is the first definition.
'monthly' is the second definition.
(thesaurus)
'that could be' is the link.
(Other definitions for magazine that I've seen before include "Periodical; arms store" , "What holds cartridge" , "Section of a firearm" , "Storehouse for explosives" , "Reading matter" .)