This is an interesting one that took a lot of work, the photograph was about 3" square, very thin paper, badly faded with a lot of spotting
I will need to do a bit more research on the troops in the picture, I'm guessing they are British, the odd style helmets were worn by some British units. There guns are short, possibly Enfield Muskatoons a type used by the British army starting in 1861, it was also used by both sides during the US Civil War.