I just built a new house with a co crete gun vault on the lower level. I want to make sure my gun are safe from high humidity. Can anyone tell me acceptable levels? I am in Michigan. (I also have a place and guns in Arizona but, that in never a problem there!)

Install an adjustable dehumidifier and set it to match what your humidity in AZ is.

Also if its newly poured concrete it will be very humid until cured of a couple of years......ask me how I know....The dehumidifier is A great suggestion.

I keep mine around 49 percent.

Why

Seems to work.

First post I see; welcome to AR!
I set my humidifier at half the range of the dial. Why? it works. No idea what the humidity is, but steel don't rust and stocks don't crack, so after 40 years, it must be right.
The more important issue is, where will the water go? I just empty the tray by hand. Although it has a hose attachment too. And it is Africa hot and humid here in the summer and colder and drier than Mongolia in winter. Not sure why I live there but that is another story.

I keep mine at around 45% though out the year