But, that happens wherever War goes. Even Kombololand.
The subway scene in Predator 2.
Another good example is In Naked Gun when Nordberg busts into the cabin on the boat. Everyone in th ecabin has a pistol.
True, but I think that particular scene fits the trope best.
Reminds me of this true story (“improved” slightly in the retelling): Gun Shop Robber | Snopes.com
This is especially a cliche in cowboy saloon scenes where someone is caught cheating at cards. Could be a trope for that specific context.
A good example of what the OP is talking about can be seen in the funeral scene in Last Action Hero:
- YouTube (around 1:00)
Includes grannies and nuns pulling out automatic weapons.
Reminds me of a story Joe Waumbaugh told about two really clueless armed robbers who tried to take down a cop bar.
It didn’t end well either. For them, at least. I think the coroner listed it as suicide. 
Interesting. I went to the Last Action Hero trope site, and this scene is categorized under More Dakka. I’d say that’s not quite accurate, and it probably deserves a trope of its own if more examples can be found.
I hate to admit it, but my wife & I watched Mega Python vs Gatoroid on SyFy the other weekend.
There was a scene at a outdoor function that the gators / pythons attacked, and as soon as the attack began, everyone (including the waiters) was suddenly packing heat, and laid down a torrent of lead.
Let us not forget “the wrong god-damned rec room.”
Page now has some basics to start with. I’m going to be working on it, everyone feel free to add your ideas. Thanks all!
ETA: and thanks to whoever cleaned up my entries and added links!
Wouldn’t that be a Wall of Weapons?
That too. But Burt and Heather always have another weapon at hand.
In the Tremors case, “everyone” was only two people, but how armed they were! 
Which would make them a pair of Walking Armories. 
This XBOX 360 ad is an amusing play off of that idea.
Closest I could find was I Like Swords, the main sword trope index.
R.H. Junir webcomic
Doesn’t this happen pretty famously in Raiders of the Lost Ark? Indy is in the restaurant in Cairo, talking to Belloq. Indy pulls his gun, and suddenly everyone else in the place pulls a gun and points it at Indy. Indy makes his escape when a bunch of children rush in and surround him, easing him toward the door.
I’ve seen this done at lease a couple of times with rappers.
Reno 911: Miami where Dangle fires a gun to quiet a rap party and all of participants pull out and aim their guns at him and Junior.
Or the Simpsons episode “Pranksta Rap” where Wiggum busts into Alcatraaz’s house with a gun and of Alcatraaz’s rapper friends take out their guns.