In fact, you would expect many differences. But the comparison is made on the overall pattern, contour, and nature of the marks that are present.

Q. Again there are dissimilar marks on these two pictures [of the firing-pin depressions on the cartridge case Commission Exhibit No. 543, and a test cartridge case], Mr. Frazier ?

A. Yes; there are, for the same reason, that metal does not flow the same in every instance, and it will not be impressed to the same depth and to the same amount, depending on the type of metal, the blow that is struck, and the pressures involved.

Q. Is your identification made therefore on the basis of the presence of similarities, as opposed to the absence of dissimilarities ?

A. No, that is not exactly right. The identification is made on the presence of sufficient individual microscopic characteristics so that a very definite pattern is formed and visualized on the two surfaces.

Dissimilarities may or may not be present, depending on whether there have been changes to the firing pin through use or wear, whether the metal flows are the same, and whether the pressures are the same or not.

So I don't think we can say that it is an absence of dissimilarities, but rather the presence of similarities.5

A bullet or cartridge case cannot always be identified with the weapon in which it was fired. In some cases, the bullet or cartridge case is too mutilated. In other cases, the weapon's microscopic characteristics have changed between the time the suspect item was fired and the time the test item was fired--microscopic characteristics change drastically in a short period of time, due to wear, or over a longer period of time, due to wear, corrosion, and cleaning. Still again, the weapon may mark bullets inconsistently--for example, because the bullets are smaller than the barrel, and travel through it erratically. 6

The Rifle

The rifle found on the sixth floor of the Texas School Book Depository shortly after the assassination was a bolt-action, clip-fed, military rifle, 40.2 inches long and 8 pounds in weight.7 Inscribed on the rifle were various markings, including the words "CAL. 6.5," "MADE ITALY," "TERNI," and "ROCCA"; the numerals "1940" and "40"; the serial number C2766; the letters "R-E," "PG," and "TNI"; the figure of a crown; and several other barely decipherable letters and numbers.8 The rifle bore a very inexpensive Japanese four-power sight, stamped "4 x 18 COATED," "ORDNANCE OPTICS INC.," "HOLLYWOOD CALIFORNIA," and "MADE IN JAPAN'' 9 and a sling consisting of two leather straps, one of