I don't know about Napoleonic rifles but when I was a cadet we trained on the Lee Enfield MK IV 3.03. Now this was probably the most powerful rifle that the British army used. IIRC it was in service in the Great War.

http://en.wikipedia.org/wiki/Lee_Enfield_No._4

Now this little baby could kill a man at 1 mile, if you were a good enough shot, and knock a man down at a mile and a half.

800 yds doesn't seem that far fetched to me, even given the fact that it was one of the earliest rifles in use.

Arn't all rifles much more powerful than muskets?