I have 7.62mm (small), 5.56mm or .50cal(large), and 20mm (very large) Ammo Boxes. Most other sizes are fitted into these three sizes, mostly. I also have a 10ga flare box and they are square shaped.

Artillary shells come in metal canisters which also make great anchor buoys for small and large vessels.
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QMC, USCG (Ret)
The best luck is what you make yourself!