Read in Irving Wiley Bell's "Life of Johnny Reb" that the "Yankees" had attached mirror(s) to their rifles so they could shoot them from the safety of their rifle pits or trenches at Petersburg.
During WW I, the British, Australians, New Zealanders and we modified our rifles as "periscope" rifles with mirrors and other complicated jigs. Kinda like a reinvention of the ACW version. You can see a couple of examples at the Springfield Armory National Historic Site.
My question to you the panel is can anybody verify Bell's statement. I looked up his cite and couldn't confirm it nor have I ever seen a rifle (1863 Springfield or otherwise) which modifications that could permit shooting from a position of concealment without exposing oneself.