Abstract: A safety device for use with a gun of a type which fires a conventional cartridge casing having an annular rim portion which cooperates with an ejector mechanism of the gun to unload the cartridges. The safety device includes a body portion having a neck portion extending from one end and a rear portion at an other end. The plug is inserted into a firing chamber of a gun with the neck portion extending into a bore of a barrel of a gun. The neck portion has a diameter slightly greater than the inner diameter of the bore to provide a press fit. The rear portion of the plug has a cylindrical portion having a diameter less than the diameter of the annular rim portion of a suitable cartridge casing such that the ejector mechanism of the gun cannot engage and eject the plug from the firing chamber. The plug is, thus, fixedly inserted into the firing chamber of the gun so that the gun cannot be accidentally loaded or fired.