The National Rifle Association explains that its NRA Guide to the Basics of Personal Protection Outside the Home "deals with concealed carry techniques and the presentation of a firearm from concealment." I am unable to understand NRA logic, expressed by NRA executive Chris Cox, assuring us that a concealed weapon causes violent criminals "to retreat into fear."
Many establishments intentionally reveal their weapons as a deterrent. They warn customers that "This site is under surveillance 24 hours every day." Businesses have anti-theft scanners in very plain sight to discourage shoplifters.
Entries to government agencies have police officers with firearms that everyone can see. If I need protection from a trespasser, I announce with a large sign: "WARNING: Attack dog on duty."
The array of security measures in airports is not hidden. The United States certainly does not conceal its Army, Navy and Air Force from potential aggressors.
If protection against physical harm is our goal, a firearm that everyone can see may very well be a deterrent; whereas, a concealed weapon can only surprise.