These are not actually guns at all, but a device lowered into an already drilled well that perforates the casing in order to allow oil or gas to flow from the rock into the wellbore, then to the surface. The “guns” are explosive devices that are “shaped charges” focusing the force of an explosion onto a very small area on the inside wall of the casing. The casing is actually “punched or pushed” to be penetrated by the explosive. The explosive will penetrate as much as five feet into the rock near the wellbore. The holes left after the explosion are called “perforations.” Each well normally has many of these perforations to allow the maximum amount of oil or gas into the well bore. This is a relatively dangerous operation but is carried out on nearly every well drilled anywhere safely. In the old days, this was referred to as “firing the well” or “blowing the well” when very brave (or not so smart) men lower nitroglycerin into the well to detonate an explosion to “bring in the well.” This method did not last long as the list of available nitroglycerine specialists grew very short very fast. Companies such as Schlumberger, Halliburton, Weatherford International, and many others supply this technology and service to the industry.