Do you know what makes an AR-15 pistol Builder is legal? Firstly, you need to remember that it is a semi-automatic handgun. So, it is a weapon for both law enforcement and civilians. The ATF has instituted a set of regulations for different firearms. In the case of the AR-15, the receiver has to be registered. You have to get an identification number and then start customizing the weapon. The pistol performs well in short spaces, but there are several disadvantages too. Despite such factors, the AR-15 gun is exceptional.