This question originally appeared on Quora. 
Answer by Tim Dees, Retired cop and criminal justice professor, Reno Police Department, Reno Municipal Court, and Pyramid lake Paiute Tribal Police Department
Unless you're in the business of supplying props to a movie studio or some similar trade, I've never understood why people want to have "replica" firearms. The only times I have come across them, I came close to shooting the person that was holding them.
The police station where I worked included a city jail for many years, and there were several bail bond offices within a block of the station. Once of them was a combined bail bond office and residence of the business owner and was directly across the street from the police station.
One of our patrol officers was returning to the station when he observed a 13-year-old outside of the bail bond office/residence, pointing what he believed to be a Mac-10 machine pistol at another kid. The officer stopped his car, took up a defensive position behind his engine block, and proned the kid out at gunpoint. It was a very tense situation, and I am reasonably sure the kid would be dead if he had pointed the gun at the cop. This was not a nervous rookie. He was a patrol officer of over ten years experience and a master sergeant in a Force Recon unit of the Marine Corps Reserve.
The Mac-10 turned out to be a water pistol. It was taken into evidence, and the kid was released while the district attorney decided whether to file charges (in the end, he didn't).
The next day, the bondsman, the kid's father, walked into the lobby of the police station, carrying another Mac-10 water pistol. This time, he raised it in the general direction of three officers (including me, having just returned from a court appearance), and sprayed the front desk with water. His intention was to demonstrate how ridiculous it was for our cop to have thought the water pistol was a real gun. He was appalled that all three of us drew our sidearms and pointed them at him. We didn't shoot him when we saw the water, but I could easily believe that another cop wouldn't have seen the water at all and would have shot him. Observation of details can get distorted under stress.
A few days later, a photo of the water pistol and a real Mac-10 was distributed, the two "guns" side by side. No one could tell the difference from the photo, which was certainly a better view than a cop would have had of the gun in the hands of another person, from a distance.
Cops react to threats, without regard to who the threat is. This is trained into us. Paper targets are available where different figures are shown holding innocuous objects like phones and hair brushes, with the objects easily replaced with guns. Here's an example:
That same target would be supplied with an overlay of a cell phone or some other non-threat object in place of the gun. These politically incorrect training aids are used to underscore the basic truth that you are just as dead, whether the killing shot is fired by a forty-year-old career criminal or a ten-year-old kid screwing around with daddy's revolver.
Other answers here state that American police are too trigger-happy. I submit that cops regularly refrain from shooting in circumstances where it would have been permissible under the law and their own agencies' deadly force policies, as we did when the bail bondsman walked into our lobby with the water pistol. I know I was about two pounds into a five-pound trigger pull when my brain perceived the water. I can't speak for my colleagues.
I would like to know who permitted the replica AK-47 to fall into the hands of a 13-year-old, and why that kid wasn't schooled on the handling of such things, especially in the presence of cops.
The argument that the area where this took place wasn't heavy with gang activity is a non-starter. Gangsters don't restrict their travels. They can show up at your church or shopping mall. You can't tell your cops to adopt a reduced level of awareness or readiness to use deadly force in Neighborhood A vs. Neighborhood B. I doubt that anyone expected to see guns at Sandy Hook Elementary School or the Washington Navy Yard, but, damn, there they were.
For those of you who are so outraged at this, consider this scenario: the cop doesn't shoot because the guy bearing the AK-47 is only 13 years old and is in a neighborhood not rife with crime. Then, the cop gets killed, because the AK-47 is a real one, and the kid woke up that morning and decided he wanted to know what it's like to kill someone. Would you feel better with this outcome? Are the lives of cops more expendable than those of homicidal teenagers?