This is an interesting one that took a lot of work, the photograph was about 3" square, very thin paper and badly faded, possibly 1860's
More research on the troops in the picture, I'm guessing they are British, the odd style helmets were worn by some British mounted 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.
Sc37,403