[FreeAtlanta]

That one had me puzzled, too. After getting creamed all thread, I was too intimidated to ask. :-)

[101stAirborneVet]

I don't know where Ayoob is getting his info, but generally what is being said here is that Zimmerman fired a single shot, but that the next round did not advance into the chamber for unknown reasons.

This can happen if someone holds the slide in place. Imagine firing a semi auto pistol, each time you fire the slide blows back and then returns forward. This action advances the next round into the chamber to be fired.

Ayoob makes the case that during a violent struggle over a handgun, Martin's hand may have been on the slide preventing the next round from loading when Zimmerman fired.

[FreedomPoster]

Just to expand on 101stAirborneVet’s answer on a pistol being “in battery” for boop -

To make an automatic pistol ready to fire, you insert a loaded magazine, pull back the slide all the way (which takes the pistol out of battery). On the way back, a round (assembled bullet+casing+powder+primer) is stripped from the top of the magazine, and forced into the back end of the barrel. When the slide is all the way back, you release the slide, and the recoil spring moves the slide assembly forward on the frame, and the barrel then locks into the slide ready to fire. There is a lot of detail left out, but that’s the general sequence of events. Once this occurs, the pistol is said to be “in battery”.

When the pistol is fired, the bullet exits the muzzle, and the recoil causes the slide to move backwards on the frame, and mechanism causes the barrel to unlock from the slide (now “out of battery”) so that the spent casing can be ejected on the rearward trip of the slide on the frame, and another round is stripped off of the top of the mag, slide reaches full rearward travel and starts moving forward, and then the barrel and slide lock up and go “into battery” to be ready for the next shot.

If something interrupts the rearward motion of the slide after the pistol is fired, it stops this from happening. It is possible that this happened with Zimmerman. Round fires, slide moves backward and the spent casing ejects, but perhaps the slide catches on an article of clothing instead of moving fully rearward properly and stripping that top round off of the magazine. Instead, the mag is left full, and slide / barrel / frame are left in an un-locked-up, out of battery configuration.

That’s probably a little difficult to understand. It’s the kind of thing that if we were sitting over the kitchen table with a firearm, I could show it and explain it in 10 minutes and you would understand it perfectly. Harder to do without props. There’s probably a YouTube out there that would explain it perfectly.