A few points, based on comments in this thread and some PM's/e-mails received from others interested in building their own ammo lockers.
First, I really don't think that this is "too much" space for ammo. Allow me to present Preacherman's Postulate For Ammunition Storage:
"Ammunition Expands To Fill The Space Available"!
I started out with one half-height office cupboard (36"x18"x36"), with three heavy-duty shelves. Within a year, I had to move it over to a full-size cupboard, as illustrated here. The one I've just built is my second!
Also, it's amazing how your firearm "accessories" accumulate... holsters, cleaning gear, ammo, tools, bags, boxes, etc., etc... I'm building this second ammo locker because once I've moved ammo over to it, neatly arranged by caliber (often in milsurp .50-caliber ammo boxes), I'll be moving the rest of my firearm odds-and-sods into the old ammo cabinet. For the purpose, I've bought a number of the small Rubbermaid plastic storage containers, in various sizes, like this:
(Walmart again!). I'm going to sort my gun goodies into categories, and box them up separately, and store these boxes in the other cupboard, so that everything is in one place, neatly arranged, and accessible whenever I need it. How many times have you looked for a particular cleaning jag, or gunsmithing tool, or whatever, and had to hunt all over for it? This ammo closet helps organize all of them too, so I don't think space will be wasted.
Finally, a couple of folks who are already building similar lockers complained that they found it very difficult to get the front cross-members in place, because the uprights were being forced into the small gaps between the door hinges and the sides of the cupboard. Agreed, that can be a problem - the solution is, assemble the front uprights and cross-members
outside the cupboard, and put them in place as a unit. Then, put the rear uprights in place, and attach the front-to-back cross-members; and finally, attach the rear cross-members. This ensures that the front uprights are held in place by the cross-members, and can't be pushed out to funny angles.
Hope this helps... I think I'll have to start a photo thread for ammo storage when this is all done!