You're watching a popular Science Fiction film when suddenly the argument you were about to launch over whether or not Han shot first is derailed by the realization that you saw the same weapon the night before being used by a German officer in the trenches, only then it wasn't firing Frickin' Laser Beams.
"Hey, It's That Gun" is where the viewer recognizes a firearm playing a role in a world one would not expect to find it. This most frequently arises in fictional, fantastic or alien settings where prop designers are looking for something realistic, functional or available off the shelf.
Not to be confused with A.K.A.-47, where Real Life weapons are simply renamed in their proper context, or Improperly Placed Firearms, where they are used in an inaccurate context but are at least still firing the same kinetic projectiles they fire in real life. This describes where a viewer is able to recognize a firearm that itself is playing a role that is accurate within the context of the work.
Just like human actors, firearms may be visually modified or simply left as is. The latter is frequently the case with Cool Guns or Rare Guns, but using something that is too common/recognizable may break the viewer's Willing Suspension of Disbelief.
Does not apply when an existing firearm is sufficiently modified as to be unrecognizable to anyone not using a third-party source like the Internet Movie Firearms Database. Will re-apply if sufficiently modified weapons are in turn used in other works where they can be recognized from their other appearances.
- Demolition Man features an HK G11 concept rifle from the 1980's playing the part of a "Neutron Accelerator".
- The Lawgiver from Dredd is a Glock 17 with a heap of extras added to it.
- The "futuristic" U.S. Police Force guns in Escape from New York are blatantly just M16s with the handguards removed.
- In the 1968 Planet of the Apes film the gorilla soldiers carry Lugers, which could be this or a clue that it's Earth All Along.
- The RoboCop franchise has a couple good example with Robo's famous "Auto-9" handgun being played by a modified Beretta 93R and the "Cobra Assault Cannon" being played by an M82 Barrett sniper rifle.
- Said famous Auto-9 handgun reappears in the film version of City Hunter as the weapon of choice for the Big Bad.
- Serenity shows the crew finding a World War II era German Flak 30/38 20mm anti-aircraft gun and strapping it to the ship's hull to pass as reavers
- The original Star Wars films were the Trope Codifier, with all weapons props being created from military surplus stocks and then visually modified to look futuristic.
- Han's DL-44 blaster is, quite obviously, a C96 "Broomhandle" Mauser pistol.
- The standard Imperial E-11 blasters were made from surplus British Sterling submachine guns.
- The DLT-19 heavy blaster rifle was a modified German MG-34 machinegun, parts of which were also used in the prequel DC-15A blaster rifle.
- The DLT-20 blaster rifle was constructed from German StG-44 assault rifles.
- Timecop armed the protagonist with a Taurus PT99AF pistol with bits added to bulk up the slide. The same weapon has since appeared in multiple other places, including a few appearances late in Stargate SG-1.
- In Total Recall (1990) and a number of other films set 20 Minutes into the Future the Calico Light Weapon System has become quite popular.
- Doctor Who: On first glance the Sontarans' weapons look like the sort of sleek, futuristic weapons you would expect them to have. Except if you play laser tag, you'll recognize the "gun" when it gets a close up . . .
- Firefly uses a huge variety of real and modified-but-recognizable guns. Mal's pistol is a Taurus 85 revolver with bits glued on to make it look like a semiautomatic. Jayne's beloved Vera was modified from a Saiga-12 semi-auto shotgun, and his sidearm is a Civil War-Era Lemat revolver with what looks like a universal mount on top of the pistol barrel and some sheathing around the shotgun barrel. Zoe's favored weapon is a Mare's Leg sawed off rifle (with a couple vaguely Space Western modifications) which first made an appearance with Steve McQueen in Wanted: Dead or Alive before being defictionalized.
- The Canaries in Red Dwarf carry modified MP5s and Glock pistols on suicide missions. The Steyr AUG is carried by ship's security personnel, but with no modifications.
- Space: Above and Beyond used Ruger Mini-14s in custom shells as the "M590 assault rifle".
- In Halo, set about five hundred or so years in the future, the UNSC apparently still uses twentieth-century South African 14.5mm anti-materiel rifles (albeit converted to semiautomatic fire and with the magazine in the bottom rather than the side of the rifle).
- Command & Conquer: Tiberian Sun has an interesting example when it comes to the "M16 Mk. II pulse rifles" both sides make use of. Both sides have two different variations of the weapon that show up in cutscenes, and while Nod's go between a completely unmodified real M16 for live-action cutscenes and a generically-boxy model for CGI ones, GDI's models for their pulse rifles are the M41-A from Aliens and the M590 from Space: Above and Beyond.
- Treasure Planet: Battle at Procyon: The Plasma Cannons appear to be Armstrong Guns with recoil hydraulics added.