Colt Made Them Equal

God created men. Colonel Colt made them equal.

—Old frontier saying, quoted by Larry Koller in The Fireside Book of Guns

After ten thousand years the Sealed Evil in a Can is finally free and thinks it's time to Take Over the World (or destroy it. Possibly both.) using its mighty power. With its immunity to swords and arrows, what hope does humanity have of defeating it?

Fortunately for the continued existence of humanity, while it's been gone modern arms development has really taken off and the once mighty forces of the supernatural are no match for the power of modern firearms. Even a Ragtag Bunch of Misfits can hope to defeat ancient, world enslaving monsters with access to More Dakka.

Common in Urban Fantasy. Contrast Immune to Bullets.

Examples of Colt Made Them Equal include:

Anime and Manga

Those Who Hunt Elves features a trio of humans from Earth tromping across a fairly standard Heroic Fantasy world -- except that one of them is driving a second generation main battle tank, owns a massive sniper rifle, and is generally a ranged-weapons badass. Cue Curb Stomp Battle after Curb Stomp Battle (Played For Laughs, usually) -- like the skyscraper-sized golem which barely gets to stand up before the tank blows it to pieces.

Literature

In Monster Hunter International the two newest members of a group of vampires, a Nazi and a former monster hunter, warn their group that while they've been sleeping modern weapons have made humanity much more dangerous to deal with and they should act with more subtly. Fortunately for humanity, the others don't take their advice and sneer at the might of humanity's "cannons".
Word of God for the Harry Potter series is that modern weapons would make short work of wizards, which is a reason for their modern isolation.

Live-Action TV

In [Buffy the Vampire Slayer]] a demon that was sealed because it was immune to any weapon "forged" proves no match for a rocket launcher.

Tabletop Games

In many game lines in The World of Darkness (e.g. Vampire: The Masquerade), part of the reason supernatural forces hold their version of The Masquerade now is that modern humanity is more than equipped to bring them down if they know. Even those that have protection from normal bullets will fall quickly to great volumes of them or heavier stuff like explosives and humans who know what they are up against will pack Silver Bullets or equivalent. The Masquerade must be tightly enforced as the modern age also brought the internet and cell-phone cameras and breaches will quickly spread.

Video Games

In Eternal Darkness many player characters cover two thousand years of history. The early ones tend to die (or live) horribly after their adventures. The ones with guns tend to live, if horribly scarred from their experience. Dr. Lindsey manages to scare off the Centurion lich Pious Augustus through use of shotgun.

Web Comics

El Goonish Shive had a moment when Pandora was ranting about the whole old hide-the-magic thing. Magic obviously could affect balance of power much while humanity was using glorified pointy sticks, but in the age of firearms (and ICBM) this consideration does not hold much water. Magic itself, however, doesn't care about balance of power, just being rare and mysterious.
- Also, Raven almost got killed by an aberration smart enough to keep the low profile and use things humans would use when it wants to do something people will notice — like Molotov Cocktail, or (in this case) a revolver, rather than bizarre magic powers.

Web Originals

One of the central conceits of The Salvation War -- In a World where Heaven and Hell are both essentially Bronze-Age civilizations inhabited by humanoids with a few tricks, both are vulnerable to 21st-century human weaponry when God abandons earth and hands it over to the forces of Hell.