Depends on what you mean by "secure." I no longer have kids at home, so I don't need to keep it "secure" from them. When the kids were still at home, ammo was kept in locked storage boxes. Didn't have as much then, as now, so that wasn't too difficult. But now I've got a lot. Most of it is kept in ammo cans, and most of those are in a locked storage room. But it isn't very secure in that the door could probably be busted open with a swift kick or too. The lock just keeps visitors or grandkids from casually opening the door to see what's behind it.
It isn't worth it, to me, to buy enough safes to keep it secure in that sense. But the storage room is inside the house, which is protected by a security alarm system, for whatever good that might do. I doubt anyone could cart it
all off before the cops arrived, but if bold enough they could get some of it.
I don't really want to purchase another smaller safe to store ammo and was looking at some lockable cabinets I could reinforce and screw into wallstuds in a corner.
Any ideas or recommendations? Maybe old file cabinets?
Who are you trying to keep it from? A prybar will likely bust open lockable cabinets easy enough. So that's not much security against a determined thief. If you are just trying to keep it out of the hands of children, it might be okay, if you don't have too much. In my storage room, I have ammo cans stacked 5-6 high, and old file cabinets would not hold up to that kind of weight. So the lock on the door keeps the casual visitor or child from mischief, the same way lockable cabinets might. Again, it is not worth it, to me, to invest in much more than that to keep ammo "secure."