First, Do not put primers in an ammo can, it becomes a bomb. No not by legal standards but by common sense standards.[/B]
As for ammo, anything larger than a .50cal can with loose ammo in it becomes difficult to move and carry, boxed ammo is ok because you can only fit about 1/3 the amount.
A 50cal can loaded with loose loaded.45acp ammo weighs around 70lbs and 9mm isn't much lighter.
A 50cal can full of just 230gr fmj bullets will ruin your back if you move it incorrectly.......I know, I've done it.
They are great for storing about anything that you want to keep out of the elements and as posted above, you cannot have and will likely never have enough of them. I loaded a van completely full of them from a place in Ohio once and thought that ought to last me for life. That was not the case and I needed more within a very short period of time.
If they're priced right, buy them, just beware of their weight once you fill them up.
They make great first aid kits, great for storing knives, camping cooking utensils, binoculars, VHF/UHF ham radios or a CB, tool box, reloading tools, as a portable or temporary gun locker if its secured down to something and can't be carted off easily, torch tips, welding wire spools(the small ones) fitting and hardware assortments, ect. ect.