A firearm safety device for installation into an otherwise functional firearm for disabling the firearm with respect to chambering and firing a round of ammunition and thereby enabling the firearm to be used as a training firearm. The functional firearm includes, among other items, a removable barrel that defines the bore for firing a round of ammunition. The barrel replacement is a unitary, solid structure wherein the portion corresponding to the barrel bore is closed, thereby preventing the chambering and firing of any round of ammunition. The barrel replacement is constructed and arranged to accurately and precisely simulate the size and shape of the removable barrel so that once the barrel replacement is installed, all other portions of the firearm remain functional. This enables other aspects of firearm usage and safety to be explained while, at the same time, providing an absolutely safe firearm that is not capable of firing a round of ammunition.