I 100% agree to remove the moisture issue first, I discovered this first hand. Don't make the same mistake. As some have said there are moisture resistant paints, don't be cheep, get good stuff and put it on heavy. Use a dehumidifier, or two of 'em if you need to. Make sure your sump pump functions properly, or put a hole in the floor and install one.
Remember that all these devises, including the paint can fail. Diligence is your only real answer for that. Also you can place some of those moisture absorbing bags in the safe, and hopefully that will protect your guns, but what about the exterior of your safe, specifically where it contacts the floor? Maybe you can raise that off the floor somewhat, or use rubber pads to isolate it.
Then there is the flooding issue. What happens when you leave for work, the house is empty and for no good reason a pipe bursts and you dont discover it for 10 hours? Or the creek rises, or you're on vacation for 2 weeks?
Honestly I don't think the basement is your best location.
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If you cannot stand behind our troops, (and I understand, no judgement here), then please stand in front of them, remember, they're willing to stand in front of you!