But is it an entirely plausible weapon? I am no weapons expert — in fact I fear to ever lay hands on one for I know I would become a gun nut, but I will confess to being a dead shot with a Red Ryder 200-shot BB gun. And I am well aware that some “air rifles” can be used to take down varmints and that Crosman sells a rifle that throws a .357-caliber slug.
If you have no idea what I’m talking about, I draw your attention to The Adventure of the Empty House, the story in which Arthur Conan Doyle returns Holmes from the dead. Colonel Sebastian Moran, Moriarty’s surviving lieutenant, has already used the air-gun, loaded with a revolver round, to kill Ronald Adair. The round had mushroomed, causing incredible violence, and yet no one reported the sound of the shot. Now he is about to use the same air rifle in his attempt to assassinate Holmes. Here we see Moran firing at the wax dummy of Holmes from the empty house opposite 221B Baker Street:
Then from the pocket of his overcoat he drew a bulky object, and he busied himself in some task which ended with a loud, sharp click, as if a spring or bolt had fallen into its place. Still kneeling upon the floor he bent forward and threw all his weight and strength upon some lever, with the result that there came a long, whirling, grinding noise, ending once more in a powerful click. He straightened himself then, and I saw that what he held in his hand was a sort of gun, with a curiously misshapen butt. He opened it at the breech, put something in, and snapped the breech-lock. Then, crouching down, he rested the end of the barrel upon the ledge of the open window, and I saw his long moustache droop over the stock and his eye gleam as it peered along the sights. I heard a little sigh of satisfaction as he cuddled the butt into his shoulder; and saw that amazing target, the black man on the yellow ground, standing clear at the end of his foresight. For an instant he was rigid and motionless. Then his finger tightened on the trigger. There was a strange, loud whiz and a long, silvery tinkle of broken glass.
Later, Holmes says:
“An admirable and unique weapon,” said he, “noiseless and of tremendous power: I knew Von Herder, the blind German mechanic, who constructed it to the order of the late Professor Moriarty. For years I have been aware of its existence though I have never before had the opportunity of handling it. I commend it very specially to your attention, Lestrade and also the bullets which fit it.”
I am unsure whether an airgun, firing a revolver bullet from across the street, can penetrate a glass window and still cause the damage to the wax dummy as described in the story. I concede a rifle powered by compressed air can easily be lethal, but I am unsure if a weapon powered by a spring — “a loud, sharp click, as if a spring or bolt had fallen into its place” — could be so powerful. Admittedly, it is described that Moran “bent forward and threw all his weight and strength upon some lever,” making it clear he was tensing some very powerful spring, perhaps with the aid of a winding crank.
I suppose there are so many variable here that it would be impossible to hazard a guess. We do not know the distance involved, the strength of the glass or the caliber of the revolver round and I am inclined to give Conan Doyle the benefit of the doubt, but I hope the Holmes Mythbusters might decide to tackle this question.