Four experts in the field of firearms identification analyzed the nearly whole bullet, the two largest fragments and the three cartridge cases to determine whether they had been fired from the C2766 Mannlicher-Carcano rifle found on the sixth floor of the Depository. Two of these experts testified before the Commission. One was Robert A. Frazier, a special agent of the FBI assigned to the FBI Laboratory in Washington, D.C. Frazier has worked generally in the field of firearms identification for 23 years, examining firearms of various types for the purpose of identifying the caliber and other characteristics of the weapons and making comparisons of bullets and cartridge cases for the purpose of determining whether or not they were fired in a particular weapon.[C3-136] He estimated that he has made “in the neighborhood of 50,000 to 60,000” firearms comparisons and has testified in court on about 400 occasions.[C3-137] The second witness who testified on this subject was Joseph D. Nicol, superintendent of the bureau of criminal identification and investigation for the State of Illinois. Nicol also has had long and substantial experience since 1941 in firearms identification, and estimated that he has made thousands of bullet and cartridge case examinations.[C3-138] In examining the bullet fragments and cartridge cases, these experts applied the general principles accepted in the field of firearms identification, which are discussed in more detail in appendix X at pages 547-553. In brief, a determination that a particular bullet or cartridge case has been fired in a particular weapon is based upon a comparison of the bullet or case under examination with one or more bullets or cases known to have been fired in that weapon. When a bullet is fired in any given weapon, it is engraved with the characteristics of the weapon. In addition to the rifling characteristics of the barrel which are common to all weapons of a given make and model, every weapon bears distinctive microscopic markings on its barrel, firing pin and bolt face.[C3-139] These markings arise initially during manufacture, since the action of the manufacturing tools differs microscopically from weapon to weapon and since, in addition, the tools change microscopically while being used. As a weapon is used further distinctive markings are introduced. Under microscopic examination a qualified expert may be able to determine whether the markings on a bullet known to have been fired in a particular weapon and the markings on a suspect bullet are the same and, therefore, whether both bullets were fired in the same weapon to the exclusion of all other weapons. Similarly, firearms identification experts are able to compare the markings left upon the base of cartridge cases and thereby determine whether both cartridges were fired by the same weapon to the exclusion of all other weapons. According to Frazier, such an identification “is made on the presence of sufficient individual