What I would call “everybody’s armed”. The trope where it turns out that in any given crowd of people (patrons in a bar, people in line at the bank, worshippers at church, etc.), anyone foolish enough to threaten them finds out the the whole crowd is armed to the teeth, down to the senior citizen with a walker, the nun, and the clown with the balloons. Most famously done in a couple of movies where someone tries to hold up the bar where all the off-duty cops hang out, but not necessarily limited to police. An example would be the Dean Koontz novel Lost Souls where an attempt to begin assimilating the townsfolk en masse at a gathering goes awry when it turns out that almost every adult in town has a gun on them, and they offer significantly more resistance than the antagonists had counted on
I know, the site’s not very well laid out. But there is a place to ask the people who actually write the tropes. Why not ask them?
Oh! Thanks!
Yeah, you can launch what is called a “You Know, That Thing Where” (YKTTW) which puts it on a list of proposed tropes for people to add examples and suggest changes, then “launch” it when it is ready.
So you want Everybody Was Kung Fu Fighting, but with guns?
I don’t think they do.
Hmmmm. thinking back, I recall reading many years ago a book by F Paul Wilson (Healer, perhaps) which included an extreme version of that. One of the planets in his future universe is called Flint, and is basically the Planet of the Anarchist Gun Nuts. Everybody is armed. There’s these aliens who are engaging in attacks where they use some kind of teleporter/stargate to send in troops who basically kill everything that moves until troops arrive to push them back through the gate. Well, they try that on Flint and literally everyone in sight short of small children instantly turns and opens fire. Their soldiers die as fast as they can be sent through until they give up and close the gate.
Reading this description, the first film to come to mind was Hot Fuzz. But, while there is an armory of firearms-related tropes listed on the page, none seems to be the equivalent of “everybody’s armed.”
Obviously, this trope would not apply in a setting where everybody is armed and is expected to be, e.g., a Western, or the Middle Ages or the Renaissance.
If swords and such count, there’s the time a bunch of robbers burst into a tavern frequented by the City Guard in Feet of Clay.
I don’t see why it wouldn’t. I’m thinking specifically of Rango, where the title character asks if anyone has guns to contribute, and they all pull out multiples, even the cute little mole (or is it rat?) girl. It’s a western, but it’s still the same scenario as the OP.
EDIT: Another example: The third Artemis Fowl, where the all of the diners at a restaurant are assassins, even the old lady (who Artemis specifically called harmless earlier).
How about Mogadishu, Somalia?
Another setting where everyone is expected to be armed.
This happened in real life to my cousin when she took a job in Texas. One day the receptionist came to work and told everybody she’d separated from her husband and within a short time, he appeared in the parking lot, just sitting in his truck. Somebody used the PA system to give a general warning that the husband was outside, and my cousin said everyone in the firm produced a gun from somewhere and rushed to the lobby to fend off the potential attack. She said she never saw so many armed people in her life.
Nothing happened, of course. Management later sent around a memo asking employees to be more discreet in arming themselves at the workplace.
It was a ceiling-fan company, by the way.
Heh, it’s a good trope. If there isn’t such a page, there ought to be. 
I think Guns In Church is the closest.
There’s a similar scene in that Trope-magnet, The Adventures of Buckaroo Banzai Across the 8th Dimension. He & his Hong Kong Cavaliers, those multi-talented fighters against evil, are booked into a dingy New Jersey club.
In the audience, a despondent Peggy Priddy tries to shoot herself; somebody joggles her elbow & she misses. At the sound of gunshot, every member of the band whips out his weapon & stands ready for battle…
I remember a justified use of the trope in the Buck Godot graphic novel PSmith. The main setting is a legally lawless[sup]*[/sup] planet. PSmith brandishes his gun and shouts at the crowd to get out of his way. Everybody immediately draws on and zaps PSmith; Godot (PSmith’s quarry) notes that that approach would have worked anywhere else.
- It makes sense in context. The planet has but one law, enforced by an extremely powerful race: That the planet shall have no law.
Heh! New Hong Kong, the planet where even the EMTs whose job it is to help people who’ve been shot go armed. 
There is a trope for “everyone has a sword” but darned if I can find it now.
Everyone Is Armed but empty.
There’s the scene in Good Omens where War turns up in a hotel on a small Mediterranean island and suddenly everyone except War and a couple of British tourists turn out to be part of one revolutionary faction or another. It doesn’t end well.