The Art of Sleights and Deception

When Holmes tells Watson that he stared into the Abyss and the Abyss revealed him the identity of the murderer, he is referring to the quote from Friedrich Nietzsche: "If you stare into the Abyss, the Abyss stares back into you.".

Sherlock says catching a bullet seems dangerous, but is actually a very safe trick. However, at least 12 people have died, performing the trick. The guns used in the trick are real and there's always a danger to using guns. Some guns have been accidentally loaded, several have been deliberately loaded, one magician used to tamp down the gunpowder with his wand and the tip broke off and killed him when the gun was fired, and one magician performed the trick in front of an audience of cowboys, one of whom wanted to see him do it again in a local bar, with little warning and the cowboys own gun.