Maybe it's just me, but I find this a fantastically beautiful object. I have to think this old cast iron cap gun was buried for many years, then dug up in this beautiful rust-encrusted state, which for me transforms it into a sort of fossilized sculpture. The hammer and trigger, though, still work perfectly, so one could I guess use it as a cap gun yet! I've looked at some old cap guns and think it is likely a Scout Single Shot Cap Gun by Stevens dating to 1890--the profile and the patterning on the handle are a match, and if one looks closely one can make out the "Pat'd" on one side, which would read in full "Pat'd June 17, 1890". 

7" long x 3 1/8" tall x 15/16" d. Completely rusted but structurally very sound and stable.