Spoiler - Technical Notes on the Ending

Towards the end he opens the Webley Mark VI and there are two live rounds and four empty chambers. There should be four fired shell casings still in the cylinder. The only way this could have happened is if prior to that scene he opened the revolver and extracted the four empty shell casings by hand and then replaced the two live rounds. Given his state of mind this is highly unlikely. This is clearly 'dramatic license' but it is the sort of minor technical point that detracts from the immediate appreciation of the movie as it plays out since it is a bit jarring to those familiar with firearms.

That the Webley fires when he drops it is within the realm of possibility but just, and I mean just. This Webley doesn't have the sort of internal safety mechanism that prevents it from being fired when dropped even if the hammer is fully back or only partially back; instead it has a 'rebound lever' which provides some protection against firing in such a circumstance. If the rebound lever was broken, or broke on impact or there were other technical malfunctions then it is just possible that the Webley would fire when dropped.