" You're only screening out people responsible enough to read the whole lease and ethical enough to not knowingly violate that clause."
We have already got 2 folks responding in this thread, who have said they would read the clause... And Ignore it, bringing and keeping their firearms on premise.
So ethics in contract law is a malleable concept it seems, subject to variances in opinion, that the tenant might have. {?}
No screening measure is perfect.
However, having a "no guns" clause will screen out anyone with a gun who doesn't want to deal with the hassle of a landlord who prohibits guns. So it will screen out a certain percentage.
Of the people who will ignore it, a solid percentage of those will be "responsible gun owners" whom the landlord will likely never even be aware that there is a personal-safety weapon on the property.
Of those who ignore the rule and do make their weapon known, such as by keeping it visible, shooting on/near the property, or doing anything that might trigger a complaint from other tenants, the clause then provides grounds for removal if they do not comply.
For a clause to be enforceable, I would have to consult a real estate lawyer as to how exactly it would be phrased, exceptions for LEOs, etc.
It's the same way I can't prevent people from brining in pets by having a "no pets" clause, but it will dissuade many pet owners, and it would give me recourse with those who violate the rule.
This is also why I don't have a "no pets" clause, I have an "all pets subject to approval" clause, and I ask for an up to date health certificate from the pet's vet, and permission to contact the vet for more information about the animal. It won't stop anyone, but it will present a hassle that many irresponsible pet owners just won't want to contend with.
Again, this doesn't stop Joe-the-tenant from picking up a mange-ridden, incontinent pit bull from the side of the road, and locking it in the bathroom to tear apart the new shower stall, but that's the risk of being a landlord.