Can you tell which of the officers pictured is holding the real gun? One of the firearms is a Beretta 9mm and the other is a replica pellet gun that was left behind by a group of teenagers playing on a roof. It's very difficult to tell them apart in print, and it's not much easier in person. The real gun is on the left.
Advances have been made in the design and manufacture of replica firearms in recent years that make it increasingly difficult to differentiate between real and replica.
Police officers are trained to react to escalating threat levels, and facing a firearm is the highest level. Police must always assume that any firearm that is produced is real. If an officer feels that their life is being threatened, then a shot could be fired. When a person points any weapon, real or replica, at a police officer, the results can be tragic.
Many replica weapons, especially those which are capable of firing pellets or ball bearings (`BB guns') are being specifically marketed to children and young people as if they were toys. The concern is that these weapons may then be put to something less than "playful" use.
Take a look at the pairs of weapons in Firearms: Real or Replica and see if you can tell which is real and which is replica. The real weapon is noted at the bottom of each page.