Modern, I'd love to go through the joint they make the 308 mini guns in. At 4k rnds per min for testing they must give out 1xf 308 brass like candy at halloween, or maybe I could talk them out of a barrel cheap.
Of old, I'd pick the colt, henry, Shiloh, springfield, factories at certain points. Or maybe one of the non gun factories that made guns for our war efforts.