“It is possible, nevertheless.” Newton Mills’ eyes shone. “With the aid of a comparison microscope and micro-photography, it can be done.
“In the first place, the spiral grooves in a gun are made by passing a narrow cutting die many times through the barrel. No metal has ever been found that will not wear. The cutting die wears. Its edge becomes rough. You cannot see the roughness with the naked eye. A microscope reveals it. This rough cutting edge imparts just such a roughness to the spiral groove.
“Since the cutting die is constantly wearing, the roughness of the spiral groove of one gun, when studied under the glass, will not be exactly the same as that of any other barrel, though cut by the same machine on the same day.
“Now, when a soft bullet is shot from a gun, the rough edge of the groove leaves scratches upon its surface. You cannot see these scratches with your naked eye. The microscope again reveals them.
“When you put two bullets fired from two guns of the same identical type under a comparison microscope, you can see them both at once and can place their scratches side by side and end to end, and you know at once that they were not fired from the same gun.
“But if the scratches match perfectly, then you know that the two bullets were fired from the same gun, and no other.”
By this time both Johnny and Drew were listening with all their ears.
“This study,” said Mills, “is sure to be of great service to the forces that make for justice. Every crook has his weakness. A weakness common to many is love for a particular gun. A man has carried a gun and used it many times. It has saved his life by taking the life of another. The gun becomes his pal, his defender. He does not willingly part with it. And in this he reveals a great weakness. That gun has left its trademark, its bullets, behind. By these, man and gun may be traced. If the gun falls into the hands of the law, woe to the crook!
“As you know,” he turned to Johnny, “we secured the bullet that wounded Rosy; also the one that was fired that other time in the studio; and the one imbedded in the wall at Ramacciotti’s old place.
“After examining these, we fired test bullets from all guns taken by the police from suspects during the past six months.