“Are they alike?”
“Yes,” replied Drew after a moment’s scrutiny.
“No,” said Johnny.
“In what way do they differ?” The detective’s eyes lighted.
“I don’t know. Let me have them.” Johnny studied them closely.
“The grooves in one are wider than in the other,” he said at last.
“Correct. In other words, there is one more spiral groove in the barrel of one gun than the other. So we know at once that if a bullet killed a man it could have been fired from only one of these guns.
“In fact the guns are of different makes. No two manufacturers rifle their barrels in the same manner. Some cut more grooves. Some cut deeper grooves, and so on.
“We have got this far,” said the veteran detective, taking a long drink of coffee, “but that isn’t very far. There are thousands upon thousands of automatics in this country, manufactured by the same company. They are of the same rifling, same caliber and all. Suppose a bullet has been fired from a revolver. It has killed a man. You think you have the gun. You wish to say to judge and jury, ‘I have the gun that killed the man. This is the gun. I will prove it to you by a study of bullets fired from it.’ In view of the fact that there are thousands of such guns in existence, of the same caliber and manufactured by the identical machinery, are you able to prove that one particular gun fired the fatal shot?”
“Don’t seem possible,” said Johnny.