Popping a cap
Green's Dictionary of Slang defines "pop a cap" as:
to fire a weapon; to shoot someone.
In recent uses, the slang meaning is clear, and often extended to "pop a cap (in somebody's ass)." For example in this citation from 2000:
F.X. Toole Rope Burns 165: If some fool of whatever color was coming to kill him, he’d pop a cap in his ass in a heartbeat.
The two earliest attested dates provided in GDoS are 1865 and 1953. The 1953 citation clearly carries the same meaning, "to shoot someone."
‘Death Row’ in D. Wepman et al. Life 118: I popped four caps through his chest with my piece.
The 1865 citation, at first glance, seems to carry the same meaning:
in B.L. Ridley Battles and Sketches of the Army of Tennessee 454: My men halloo out: ‘Lie down, Mr. Govan, Mr. Bate is now gwine to pop a cap’.
Strange early uses
Looking through early uses of "pop a cap," I found some strange instances that made me wonder if they actually are referring to something other than firing a gun, or shooting someone. These two stood out in particular. First, from 1874:
A son of Col. Thomas, a little fellow of about seven years, had gone over to the Institute, and while there was a boy of thirteen or fourteen years of age, who had a shot gun in his hands, pointed it at the little fellow and pulled the trigger. It was his intention to pop a cap at him, having no idea that the gun was loaded.
In this first example, the phrasing suggests that the shooter had not intended to actually fire a bullet, but rather only to "pop a cap at him."
After Dixie was played Governor Gordon was called for by the crowd and came forward to acknowledge the compliment. He said the gun was loaded for to-morrow, and he would not discharge it to-night. "Well, then, pop a cap," said a voice in the crowd. The Governor said it was already popped, and thanked them again for the serenade.
Once again, this text suggests that "popping a cap" is an alternative of sorts to shooting, or "discharging" a gun. Furthermore, the phrase "it was already popped," while possibly meant as a joke, seems to make the context even stranger.
In these early uses from the 19th century, including the early attestation provided in GDoS and the two example clippings above, what is meant by "pop a cap?" Does it still refer to firing a gun in all cases? Does it mean something different in the two cases mentioned above?
Finally, if "pop a cap" meant something else in the 19th century, how did it evolve into the 20th century meaning, referring simply to shooting somebody (or something)?