i looked into this for so many hours and it is not possible to place weapons on you flood maps. And the only way is by ordnance which ruins the fact of finding weapons on maps. if there is a way possible that i am missing then post below. and if you think that they should alow you to place weapons post below.
I just posted about the same problem, made a huge map all for nothing. But yeah, 343 are trying to shove ordnance drops down our throat a kill off weapon on map by default.
I made a mpa on REACH where you had to scavage for guns nand now i cant do maps like taht anymore D: