Test for a "Recently Fired" Gun?
Not surprisingly, this turns out to be another factoid. First, a quote from a standard forensics text:
When a firearm has been discharged, if its barrel is not carefully cleaned afterward, there will remain traces of sulfate and potassium salts in the case of a charge of black powder, and traces of nitrates and sulfates when the charge was smokeless. In addition, there will be traces of the primer. . . .My student also called an expert in the Wisconsin State Crime Lab. The following is his report:
However, it is practically impossible to say how long the residue left by firing of powders (or primers) has been in a barrel. . . .
Therefore, if the firearm has not been carefully cleaned after firing, all that can be said is that it has been fired. (Frank Lundquist, Methods of Forensic Science, Volume I, (1962) p. 628.)
In an April 3  telephone interview with Regh Templin, an expert in firearms and tool mark analysis at the Wisconsin State Crime Lab for over 17 years, Templin stated that there is not now and never was a test to administer on a firearm to give an approximate time period as to when a firearm has been fired. He stated that analyzing swabs taken from a firearm can detect gun powder, but nothing more can be drawn from such analysis. Further, he stated that even administering this test to a firearm thought to have been used in the commission of a crime is useless because many gun manufacturers test fire their guns before they ship them out for distribution. Therefore, if a firearm is found to have traces of gun powder, these could be remnants of factory test firing, or an owner of the gun firing the weapon five years, five weeks, five days, or five hours before the alleged crime took place, rendering such a test useless.So the claims of conspiracy books can be filed in the category "buff forensics," the principles of which are radically at odds with real-world forensic science.