An improved firearm safety device for handguns is disclosed. The firearm safety device (10) prevents accidental firing of a gun by children, but is easily removable by an adult in 5 to 10 seconds, even in pitch darkness. The firearm safety device includes a lock sleeve (26) having an expandable end plug (30) and a lock rod (24) having a removable tip (34), which are inserted through muzzle (16) of a gun barrel (14) into an empty firing chamber (20). Removable tips, having different diameters, provide the proper expansion for different caliber handguns. A knurled cap (22), connected to the lock rod, is threadedly attached to a knurled enlarged end portion (28) of the lock sleeve, forcing the expansion of the linear slotted (32) end plug, preventing removal of the firearm safety device from the handgun. Since the safety device is able to rotate freely in its assembled position within the handgun, any attempt to unthread the knurled cap without holding the knurled enlarged end portion of the lock sleeve stationary, will be unsuccessful.