One may well consider a designated safe just for ammo. My ammo safe has walls the thickness of my Glock, solid steel, and is 3 feet square. It sits atop another safe I keep gold, jewelry and important papers in. The lower safe weighs 5,000 pounds and is moved via forklift. Forklift stacked the two safes together, that now sit 6'7" tall. Naturally, I siliconed the two safes together. lol
Price I payed for these two safes? Free. Look around, older (better) safes get left behind. Contact a business realtor, auction house, lock smith.
The top safe is full, of many ammo cans, wish the bottom safe was full. lol
IMHO, it is not prudent, to store a cache of ammo, or firearms, outside a safe. Pretty good video I discovered on new safes.