A blog named Odd Russia has photos of what they claim are a French and Russian bullet that hit each other during the Crimea War.
Odd Russia has many more photos but no link to the source of the images. It could be true, or it could be a hoax. I think both the French and Russians both used muskets or rifles of caliber between .60″ and .70″ and I do not know enough about the ammunition at the time to say if the mushroomed bullets look authentic.
Thanks to Jim for emailing me the link.