Continuity mistake: At the end of the film, in the bridge confrontation between Holmes, Blackwood and Irene Adler, Lord Blackwood forces Adler to the edge of a precipitous (and deadly) fall into the Thames. There is even a perspective shot, looking straight down, of her heels teetering on the edge and nothing between her and the river except two sailboats far below. There are obviously no scaffolds or other means of obstructing her fall in this shot. It's a dead drop. Then Blackwood shoves her straight backwards over the edge. But, when Holmes looks over moments later, Irene Adler is safe and sound on a convenient bit of scaffolding that magically appeared just a few feet below.
Factual error: The scene in Piccadilly Circus shows the Shaftesbury Memorial Statue, but this film is set in 1890 and that statue was only unveiled in 1893.
Answer: The gun was modified to where it would not fire a bullet. It remained in the barrel.
MasterOfAll