In order to unlock the Broken Bottle (and hence get a single kill with it) you'll need 5 kills with single use wine bottles that can be picked up during a round on the three new infantry focused Apocalypse maps. These wine bottles DO NOT respawn after they have been picked up and switching weapons will force you to drop the wine bottle and you'll lose it.
EDIT: Using class based gadgets also causes you to lose the bottle.
EDIT: The wine bottle has an incredibly generous hit box so don't be afraid to swing for someone if you're slightly farther away than standard melee range. The bottle will kill in one swing so go nuts with it.
After combing the maps in a private server I've got a few screenshots (click on the imgur links beneath them if you want to enlarge them) to help you more accurately pinpoint where the wine bottles spawn. This is by no means a comprehensive list (I've almost certainly missed more than a few), and if you happen to come across any more please let me know so I can update this guide.
(Also, a quick side note. The wine bottles appear to suffer from really bad pop in so they won't actually appear until you get close to where they are).Caporetto
I've managed to locate 6 bottles on Caporetto so far, so this map might be the most ideal for getting kills.Near Flag A:
Down the hill from where the Liven's Projector is located you'll find a trench with a fire/torch in it. The wine bottle is sitting on the ground in the area near the fire. https://imgur.com/kPzZ5yUNear Flag B:
This one is sitting beneath a bench with a gramophone placed next to it. https://imgur.com/PrPN75YFlag B:
Sitting on a wall in the immediate vicinity of the Liven's Projector. https://imgur.com/FMa6RzfFlag C:
Next to the bridge, slightly embedded in the ground. https://imgur.com/yrlw7e1Flag D:
On a wall near the detonator for the Liven's Projector. https://imgur.com/tdX9QeYBetween Flag D/E:
This one is sitting on the back of the train atop the viaduct. https://imgur.com/GJN2Bu8Passchendaele
Passchendaele has probably got more bottles hiding, but due to the colour palette of the map they blend in very well. I've managed to locate 4 so far.Near A Flag:
As you make your way out of the church into the trenches in the direction of Flag C you'll find the wine bottle sitting off to one side. https://imgur.com/mmKRzo5Flag B:
In the open trench area outside the bunker. https://imgur.com/omAuyBWNear Flag C:
This one is a little bit tricky and hard to see. From Flag C in the direction of Flag A you'll find a small enclosed area. The bottle is sitting on the ground. https://imgur.com/x7OrcUHNear Flag D:
Looking from Flag D towards Flag E and the German Empire spawn you'll see two destroyed bunkers. The one on the left houses the wine bottle on the floor. https://imgur.com/6MKXgcORiver Somme
So far I've managed to locate 5 bottles on River Somme. They are all fairy evenly distributed across the map.Flag A:
In the barn in a corner on the ground floor. https://imgur.com/MC7bpJyFlag B:
In the windmill on the ground floor. https://imgur.com/kuskWoZFlag C:
In the house next to some rags on the floor. https://imgur.com/ooBedI9Flag E:
In the sugar mill ruins at the base of the machine with the ladder on it. https://imgur.com/otgeSaaNear Flag F:
Across the train tracks, in the direction of Flag D, in a dug out with a German Empire flag hanging from it. https://imgur.com/LRBYYsL
As I've said before, there are probably more bottles out there that I've missed. If I have, please let me know so I can add them to this list.