In the closet where I keep my ammo boxes, I use two Akro-Mills wire shelves 18"D x 24"L x 54"H. They fit in the left and right corners of the closet nicely and I sought out these specifically because they have a very high weight capacity both for the rack and for the individual shelves (2000lbs / 600lbs per shelf). 4 shelves in the unit, plus a bit of storage under the bottom shelf.
Each shelf will fit 3 or 4 Plano or MTM dry storage ammo holders (depending on size/design), or six of the smaller metal Federal 1000 round 223 / 50 cal ammo cans.
Only issue I have with this approach is (issue with all wire shelving) the plastic ammo cans have little edged feet that get caught in the shelf wire when dragging them across the shelf to get them out. I've got some of the shelf liner mat stuff to put on these that'll solve that issue but have never gotten around to doing it.