In Survival Mode on Voyager (Owl) hardness, found a box of ammo under the blue chair behind the fire place in the Grey Mother's house in Milton. Had to spend 3 hours to chop up the chair into wood with a hatchet, but once that was done, a whole box of ammo was sitting right there in a location otherwise unobtainable. Not sure how many item spawn points are hidden underneath furniture, but there you go.