dude..yes...anything can be painted.
scuff up the ammo boxes with 200-400 grit sandpaper, prime them with automotive sandable primer, hit them again with 600 grit wet sand paper with water, them apply Duplicolor car paint(comes in rattle cans), wet sand in between 2 coats with 1200 grit, apply one last coat of color than a few layers of clear coat (Duplicolor rattlecan brand as well).
I know because I have done this for work before. My company does POP displays and we did about 100 of them like that.