Military ammo cans inside of a cheapo Stack-On "safe", actually a cheaply made locking cabinet. The 18 gun version that walmart had on sale easily fit over 20,000 rounds of 5.56 before I pared down my stockpile so new shooters could have something to shoot when ammo went scarce a while back. As I get older I shoot less as well.

The ammo cans stack great, and will keep it contained in the event of a fire and keep moisture out, the cabinet is locked to keep the kids out of it. It could be pryed open in less than 60 seconds with a stout screwdriver, but it works great for my purposes. For a stronger thing, they make an actual safe, or get a tool box as noted upthread.