It all depends on how much ammo you have. I am up to over 80 full ammo cans so a safe or cabinet is out. I have one pallet with 12 cans per layer and stack them 5 high. Takes care of 60 cans, approximately 1800 lbs. I saw a set up a member of this forum had that looked great. It was a metal four shelf open shelving system. I couldn't believe how many full ammo cans he had on it. I sent him a PM and he said each shelf was rated at 600lbs. It was cool because you could access it from any side. That was about a year ago.
I was at Sam's club yesterday and saw one like it on display. Four shelf, I think it was 72' high, 48" long and 18" deep. It was rated at 2400lbs. 600lbs per shelf. I never saw anything so sturdy. I was going to buy it but I could not even budge the box on the top of the stack. I have to go back for it tomorrow with my son and some of his friends. Only $78 or $79.
I have two large gun safes so my firearms are secure and I really don't worry about anyone getting at the ammo.
I know that didn't really answer your question but I find if you get a small cabinet your ammo gets all disorganized and it takes time to find things if you have to move things around to find what your looking for + they cannot handle much weight.
If you have a little space you might be better off picking a corner of the basement and wall in the two exposed sides, put a door with a lock on it and keep the ammo on a pallet. Probably as secure than a metal cabinet. Good luck.
Edited to add.
I don't know if you can get these unless you have a archive feature. These are the metal shelving systems I talked about.
archive.ar15.com/forums/topic.html?b=1&f=5&t=469707Also go down 20 posts
archive.ar15.com/forums/topic.html?b=3&f=16&t=278443&page=1