It certainly is. I did the chamber cast and my research points toward it being a 400 Eley, which was one of the very early coiled brass cartridges available in England. From what I could find about the maker, I believe it was manufactured in the early to mid 1860's. Someone with a bit more knowledge or information on proof marks may be able to come up with a more definitive answer.
It is certainly a dainty little thing.
Regards
LesLeeSpeed