Handguns were not issued items to infantry privates.
The early war (1861) line infantryman was as adventurous and obsessed with gear like our couch commandos are today. First long march and a lot of things were tossed including big knives, revolvers, clothing and other non issue items. After the first battle where they were proven useless , the bullet proof vests (they weren't) were tossed. The lighter the better.
I suspect that a lot of guns in the tintypes or other images belonged to the photographer's studio. There's an image of Geronimo with a Dance revolver. I doubt he ever used one.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!