What did "pop a cap" mean, other than "shoot someone," in the 19th century?

Drawing on the explanation in Hot Licks's answer and examining further uses of the phrase "pop a cap" in the early 20th century, a possible picture begins to form of the transition between the meanings.

We can call the description provided by Hot Licks "the earlier meaning."

discharging a cap-style firearm with no powder in the chamber, so as to produce a sound without actually firing a bullet.

And the meaning that involves firing a bullet might be called "the contemporary meaning."

In the early 20th century, there seems to have been a mixture of the two meanings. Often "pop a cap" was used in hunting terminology; At least, this context appeared to be common in searches of U.S. newspaper archives.

One notable aspect of many of these cases that use the contemporary meaning is that they are negated. In other words, there seem to be numerous cases where one says, "I didn't pop a cap," meaning I didn't shoot game. The negated meaning makes sense as an earlier version of the contemporary meaning because it ultimately applies to both meanings. "I didn't pop a cap," implying that one did not fire at a duck, for example, could mean "I didn't even take the first step toward shooting.

"He said the ducks were flying high and far off. He didn't pop a cap."

• 1943 - Amarillo Daily News (Amarillo, TX) 3 Nov. 7/1

Another use appears to be similar, but it is followed by context that I don't claim to fully understand.

Meanwhile, the dove hunting season rolls on, and we haven't had a chance to pop a cap, or otherwise use that two-dollar-and-fifteen-cent piece of paper.

• 1950 - Abilene Reporter-News (Abilene, TX) 15 Sep. 20/1

I can't tell what is meant by this piece of paper; Perhaps this is another slang expression of the time, or a meaning well-known in hunting circles.

Other instances of the negated use occur in military writing:

he and his lieutenant dragged a Marine gun up to the front and then didn't pop a cap.

• 1937 - El Paso Herald-Post (El Paso, TX) 10 Aug. 5/1

My hypothesis is that over time, the phrase transitioned from the earlier meaning to the negated meaning that implies neither popping only a cap nor firing a weapon, eventually into use with the contemporary meaning non-negated, where it implies shooting a weapon.

Here is a source with a meaning not explicitly negated, but still referring to "pop a cap" as being a prerequisite of sorts to firing a weapon fully.

Because there was not a line in the story that even intimated the warships did as much as pop a cap.

• 1941 - *El Paso Herald-Post (El Paso, TX) 15 May 4/2

Finally, here is an instance where the contemporary meaning is not negated at all.

Guns? They still have their uses, Hiner said, describing his plans to "pop a cap" Monday on the first day of dove season.

• 1954 - *Abilene Reporter-News (Abilene, TX) 2 Sep. 30/8

These selections were chosen to illustrate the hypothesis that the contemporary meaning grew in use out of the negated sense. I don't claim that the cited dates form any sort of boundary around which the earliest or most recent use of either sense caught on or faded away.

Given the range of interesting appearances of the phrase "pop a cap," there could certainly be a more thorough investigation into its evolving meaning and its various idiomatic uses. For instance, all the sources cited in this answer are from Texas, but as noted in the question and elsewhere, the earlier and contemporary senses of the phrase are not limited to Texas. Perhaps the phrase caught on in Texas culture from around the 1930s to '50s. Either way, there appears to be some overlap in the meanings of the phrase and a gradual evolution into the meaning we think of today, related to "shooting someone."

An Elephind search of multiple newspaper databases turns up an instance of "popped a cap" from 1849 that seems to involve shooting live ammunition at a partridge. From "Abe Eaton's Visit to the Celebration," in the [Ebensburg, Pennsylvania] Mountain Sentinel (October 25, 1849), reprinted from the New York Spirit of the Times:

All the accident that happened was going up the hill by Lew Cusgrave's thar, close by Old Pike's, the Quaker, when some of Lew's boys popped a cap on an old smoke pole at a patridge, when h——ll, Doctor, how she [Abe's mare, hitched to his wagon] did rip! She cavorted right smart but it was no go—she ony broke one try pin, and arter gitting up the hill I fixed that, and we proceeded tolerably well to the borough.

It wouldn't make much sense to fire a blank at a partridge, so I think the likelier situation being described is that the "old smoke pole" was a musket, and "popping a cap" involved firing birdshot from it.

A subsequent instance from an untitled item in the [Columbus, Texas] Citizen (September 22, 1859) likewise shows every sign of using "popped a cap" in the sense of "fired live ammunition":

A couple of young gentlemen from the neighborhood of Freisburg, Mr. James Muckleroy and Dr. McLanahan, exhibited their skill in the use of pistols, in our town, on Monday last. Getting into a quarrel, the Doctor popped a cap at Mr. Muckleroy, who returned the compliment by discharging two shots at the Doctor. We are glad to say that neither of them was hurt. Mr Muckleroy, we understand, has stood trial and been discharged. We do not know who was to blame for the affair.

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With regard to the phrase "lie down; [X] is gwine to pop a cap," which is mentioned in Jonathon Green's dictionary of slang as the earliest mention of "pop a cap," dating to 1865, that longer phrase seems to have been a catch-phrase during the Civil War. Here is an interesting discussion of its origin, from "The Soldier's Haversack," in the Edgefield [South Carolina] Advertiser (April 3, 1867), reprinted from The Land (March 1867):

An expression was very common in the [Confederate] Army of Tennessee, without any one being able to explain where it started. A soldier sends us an explanation of it from Warrenton, Va.

When R's brigade of cavalry was firt organized, it contained some rather excitable individuals, who were accused by [Nathan] Forrest's men of making more use of their spurs than of their swords. The brigade, eventually, vecame distinguished for its gallantry ; but at the time of which we speak, it was a laughing-stock not merely to Forrest's veterans, but to the Yankees themselves. One day on a train in Mississippi, the rear car was occupied by soldiers cracking their rough jokes upon one another. When their fun was at its height, a very ungainly looking "reb," with clanking spurs, long uncombed hair, and a general appearance of long disuse of a "biled" shirt, stalked to a window and thrust his carbine out of it. Then he stopped, and looked around, as though apprehensive that he was doing something wrong, and inquired, "is thar any of R's men aboard this shebang?" No answer. "I say, boys, does any of you belong to R's cavalry?" At length, some one spoke up, "I belong to R's cavalry." "Axin yer pardon, stranger," said the uncouth individual, "my old gun is dirty and I wanted to clean her out, I'm jist gwine to pop a cap. Don't be skeered, honey!"

From this, started to the taunt so often used to cowards, "lie down, I'm gwine to pop a cap."

Whether the cap in this story is merely a blank or (as seems more likely, since the asserted purpose of firing the carbine is to clean it out) a live round is somewhat irrelevant to the anecdote. The point of the insult is that the person being told to lie down is likely to be frightened by hearing a gun go off and needs to be warned in advance that it is about to happen.

In any event, the antebellum instances of "pop a cap" from 1849 and 1859 suggest that "popping a cap" involved firing birdshot or a bullet at a target, and not merely detonating a percussion cap (as detailed in the answer by Hot Licks).