Context is huge. Is he wearing a mask? Is it cocked and raised? What are the state's open-carry laws?
I voted Do Nothing because I'm assuming the context is regular open-carry, I'm in a place with a huge gun culture and it doesn't bother me one bit.
If there's one psycho with a gun, hopefully there's multiple good people with guns, and we don't have to call the people with the guns to come and shoot the psycho with the gun. Which is basically what calling the cops is.
Now, if someone was masked-up and not fucking around I would be bolting out of there like an Olympian running track.