Sadly, there isn't much to do about the mentally unstable if you are in the business of renting firearms. You cannot prevent what you don't know about.
Now, I have heard plans like denoting the legal ability to purchase firearms on one's driver's license (yes if not a felon, mentally ill, or prohibited by the Lautenberg amendment, etc). That might work, but the specifics may get fuzzy.
Now, I have heard plans like denoting the legal ability to purchase firearms on one's driver's license (yes if not a felon, mentally ill, or prohibited by the Lautenberg amendment, etc). That might work, but the specifics may get fuzzy.