From a small collection of photos of the same troops that took a lot of work to clean up, 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, they appear to be British, the sergeant stripes and odd style helmets were worn by some British units. There guns they have appear to be Enfield Muskatoons a type used by the British army starting in 1861, it was also used by both sides during the US Civil War.