Universal Ammunition

An odd phenomenon in some video games with guns in that any ammunition you pick up works with either any gun whatsoever or an entire class of guns. In the real world, cartridges have a variety of different calibers, lengths and propellants based on a number of factors, including desired stopping power, target penetration, and the amount of recoil and gas pressure needed for a semiauto or automatic weapon to cycle itself (or, the other way around, prevent it from exploding).

These cartridges are then loaded into clips and magazines designed for specific weapons, so that even if the cartridges are identical between two or more guns, you'll have to unload and reload one painstaking round at a time before you can take advantage of it; nevermind the question of where you're getting the empty magazines from. Sitting down and filling up mags is "not always an option" in a pitched firefight, and largely qualifies as an Acceptable Break From Reality.

In fictional settings, such as science fiction, this is sometimes purposely justified by declaring there has been a worldwide standardization causing an actual adoption of universal ammunition or some sort of literally universal ammunition that's capable of reconfiguring itself to completely differing weapons.

Note that most Energy Weapons, in contrast to the panoply of batteries in Real Life, are portrayed as universal. Just pop in a random battery and they're good to go.

Before adding examples please keep in mind that a lot of weapons from video games are based on real-life weapons. When two weapons which use compatible magazines in real life use the same ammunition in a game it is not an example-- though if those two weapons use different ammunition in the game, it is an inverted example.

See also Bottomless Magazines, One Bullet Clips.

First-Person Shooter

 * Golden Eye 1997 has two main types of ammo, pistol and rifle. No matter what pistol or submachine gun you use, it'll take the same ammo. Likewise, no matter what rifle you pick up, it'll take whatever rifle ammo you have. However, there are unique types of ammunition specific to certain weapons such as rocket ammunition for the rocket launcher or shotgun shells for the shotgun.
 * Medal of Honor: Allied Assault is in the same boat, with wildly different guns from four nations all accepting the same carts and mags if they're the same "type" (pistol, rifle, etc.).
 * The original PSX game averted this for pistols and sub-machine guns (for instance, salvaged MP40 guns would not give you ammo for your .45 Thompson), but played it straight with rifles.
 * Perfect Dark added a category for SMG ammo, but otherwise retained the same ammunition setup as in Golden Eye 1997. Even funnier is that there are some alien weapons in the game with draw from these pools (and some of them even have reloading animations of putting an extra glob of goob into the gun!) - good thing the Skedar chambered their handguns for 9mm, right?
 * Time Splitters also used a similar system (since the game was developed by the same people as Golden Eye 1997).
 * In Team Fortress 2, picking up any gun dropped by a dead player gives your current weapon more ammo. Even if it's something like the Pyro's flamethrower or the Soldier's rocket launcher. Even scraps of a destroyed engineer building converts into ammo.
 * It will also refill the spy's cloak meter and provide the Engineer with extra Metal for buildings. The latter makes sense while the former... really doesn't.
 * And that wooden bats and broken bottles not only provide metal, but just as much metal as the Heavy's enormous minigun.
 * An exception is made for the Sandvich and it's variants, who simply heal the player that collects them. Not the case with the Scout's energy drinks or the Jarate.
 * Lampshaded hilariously in Poker Night At the Inventory. According to the Heavy, his minigun uses ammo with classified diameters so that his enemies cannot use it... but it can use theirs?
 * Averted to a slightly unrealistic degree at times in the Call of Duty series. Even the normal and sniper versions of a weapon did not have interchangeable ammunition; while in some cases like the Springfield this was accurate since a scoped Springfield cannot reload using stripper clips, it was even the case when the weapons should have used exactly the same ammunition loaded exactly the same way.
 * Call of Duty 4 would allow you to take ammo from any dropped weapon if it fired the same kind of bullet as what you already had, and if it isn't already the exact same gun you're using, the gun itself would stay on the ground in case you wanted to swap for it. You can also increase the amount of maximum ammo for a gun by carrying another one that fires the same kind of bullet (such as the 5.56x45mm M16, M4, and/or G36C, which falls into this when you're using two guns that are clearly not using the same magazines). Singleplayer mode in Modern Warfare 2 and Black Ops are a bit stricter, where you can only take ammo from another gun if it's the same base model (i.e. an M4 with any attachment would give ammo to any other M4, but not to an M16; this is usually not very limiting because most of the Ranger missions in MW2 have infinite-ammo crates every fifty yards or so, and Black Ops likes to arbitrarily give you entirely new guns or replenish what you have when the plot calls for it). In multiplayer however, extra ammo only comes from exact copies of the gun you have, meaning that if you were carrying an AK-47 with Red Dot Sight and Red Tiger camouflage, you can only pick up ammo from another AK-47 with Red Dot Sight and Red Tiger camouflage. Basically, your only realistic hope for more ammo is finding weapons you dropped yourself or taking guns off of people you kill.
 * Played straight with the "Scavenger" perk in Modern Warfare 2 and Black Ops - every casualty will drop a backpack with one magazine/explosive for whatever guns you're carrying, another piece of your primary equipment, and one special grenade. In MW2, this backpack also contains a 40mm grenade or four shotgun shells if your gun has the grenade launcher/shotgun attachment. Yeesh, good thing the enemy didn't know they were carrying all this stuff...
 * Turning Point: Fall Of Liberty suffered from a glaring lack of ammo commonality. No two of the game's many 9mm weapons could share ammo. In fact, if you had the scoped and unscoped versions of the same gun, they still do not share ammo. Did we mention the game was an Obvious Beta?
 * In Enemy Territory Quake Wars, the Strogg use the exact same pickup for all their ammo and their health restoration. This would normally fall under Justified Examples... except that the pickup in question was established in Quake IV to be a specially treated slurry of liquefied human remains, and could not conceivably function as ammunition for anything except a particularly disgusting squirt-gun.
 * Left 4 Dead justifies this: all guns reload their ammunition or get upgraded ammo from the same pile/box, but they are clearly shown to contain a variety of ammo. There are also a few weapons that cannot be reloaded from the ammo piles but can get explosive or incendiary ammo, specifically the grenade launcher and M60.
 * Occasionally inverted or played straight in Far Cry, for example the OICW and AG36 grenade launchers do not share ammo, which they should. However, the MP5 and P90 use the same ammo, with boxed "SMG" ammo found in the game having "9x19" written on it. Right ammo for the MP5, but absolutely wrong for the 5.7x28mm P90.
 * In Far Cry 2, all weapons of the same type use the same ammunition, even when taken from picked up weapons. This means, for example, the pistols are simultaneously chambered in .45 ACP, 9mm Makarov and .50 Action Express. They apparently tried to justify this for the primary weapon slot, where the assault/battle rifles all use 7.62mm bullets in real life, even though one of them (the AK-47) doesn't use the same 7.62 bullets as the others.
 * Half-Life's pistol and submachine gun use the same ammunition. This isn't a problem with regards to the Glock and MP5, using 9mm pistol ammo, although Fridge Logic arises in how Gordon transfers the ammo between guns (he likely does so the same way he transfers bullets between half-used magazines - that is, his HEV suit does it for him). However, the Hi-Def pack - an overall graphical upgrade - changes the Glock into a Beretta and the MP5 into an M4 assault rifle. That creates a new problem - the M4 uses rifle ammo, not pistol ammo. This was fixed in the Play Station 2 port, which used the hi-def models and added a separate ammo type for the M4.
 * Bioshock 2 features various Rapture citizens carrying around the world's first custom-made .50 BMG Thompsons, so that their weapons can share ammo with Subject Delta's huge Gatling gun. Of course how the hell they could actually use a .50 BMG Thompson is never dealt with.
 * Averted in STALKER,where there there are some guns that use unique caliber suitable only for them (not to mention, each gun has three different types of ammo for various situations!).
 * Although a criticism with this is the 9x19mm (9mm Parabellum) looks a little too similar to the 9x18mm (9mm Makarov), both in the in-game name and icon. Some players not used to this may wonder why they can't use one or the other when they're both 9mm. The game does occasionally make up for this with alternate versions of guns chambered for a different, more plentiful ammo type (such as a unique MP5 firing 9mm Makarov).
 * The pistol and minigun in Unreal and Unreal Tournament share ammo; likely in reference to this, both of their Secondary Fire modes just increased firing speed at the cost of accuracy. Later games gave the two weapons separate ammo pools, first by changing the pistol into an assault rifle, and then later making the minigun fire Tarydium shards rather than conventional bullets.
 * Exhumed (a.k.a. Powerslave) has a variation in the Sega Saturn and Play Station versions in which the ammo pickups are all generic blue orbs that refill whatever weapon you are holding at the time, but the weapons themselves all have different ammo pools beyond that.

Platformer Games

 * In Duke Nukem: Manhattan Project, you have 3 different Ammo Slots: Ammo, Pipe Bomb, and Energy. Ammo is used for your Desert Eagle, Shot Gun, and Assault Rifle, though the minor pickups only show 3 Desert Eagle Rounds (The Major Ammo Pickups show a Deagle, Shotgun, and AR round though). Pipe Bombs are used for Pipe Bombs and the RPG. Energy is used for the De-Mutator (a glob throwing weapon that demutates any mutants you run across) and Pulse Rifle (almost the same as Doom's Pulse Rifle)

Real Time Strategy

 * In Star Ruler, all ships draw from an abstracted ammo supply with no attempt to distinguish types. Given that you can have any number of ships in varying sizes from fighters to planet-sized (or bigger!) super-battleships, trying to manage individual ammo types would be a real pain in the arse anyway.

Role-Playing Games

 * In Xenogears, you have to buy ammo for The Gunslinger Billy, but you don't need to change this if you equip a new gun for him.
 * Irvine in Final Fantasy VIII has ammo for his Limit Break, but these work for all the guns he can equip in the game.
 * Averted, since you custom-make all of his weapons yourself, with the exception of his starting weapon, which the first upgrade is created from by using it as a base. In this case, you're modding the rifle to the ammo, most of which you can make yourself as well. Double averted?
 * All weapons in Fallout that use the same caliber take the same ammunition. The issue of different cartridges in the same caliber is ignored, magazines/clips/belt links don't need to be kept track of, and characters seem to load mags in the heat of combat.
 * Possibly justified by the fact in a world with definitely limited ammunition guns would be modified to accept the most common types.
 * Fallout 2 has a car that can be recharged by both Small Energy Cells and Fusion Batteries - themselves used as clips for a wide assortment of energy weapons.
 * Due to a bug in Fallout 2, the P90 Expy gun was originally loaded with 9mm ammo, which was extremely rare and only (designed) to be used in this one gun (it wasn't even the only specialized 9mm ammo, either - elsewhere in the game is a Mauser that takes 9mm (ball) ammo). However, once emptied, it then took extremely common 10mm ammo.
 * In Fallout 3, the .32 Caliber pistol and Hunting Rifle both use .32 caliber ammo, however the rifle is much more powerful than the pistol. The pistol is probably the weakest weapon in the game.
 * Justified in that the rifle has a much longer barrel and a completely sealed chamber, and therefore can propell the .32 slug better then the tiny, unsealed revolver.
 * Also in Fallout 3, Lincoln's Repeater (an authentic Henry Rifle) uses the same ammo as .44 revolvers. Even one of the fans notes on the Fallout Wiki that Henry Rifles can't use modern .44 ammunition without blowing apart.
 * All firearms in Arcanum, barring a few exotic examples, take the same bullets. Larger guns just shoot more than one at a time. Of course, the Bullets schematic only requires the ingredients for black powder, so it's unclear just what exactly you're shooting.
 * On a larger scale in Eve Online, one unit of a certain "size" of projectile ammunition can be chambered in up to five calibers. For example, one unit of "Large projectile ammunition", meant to be fired from battleship guns, can be used interchangeably in Dual 425mm, 650mm, 800mm, 1200mm, and 1400mm guns. Also, a unit of ammunition unloaded from a 800mm autocannon takes up the volume as ammunition unloaded from a 1400mm howitzer.
 * Heavily averted in 7.62mm High Caliber, with each gun taking the proper caliber and many calibers coming in multiple brands and types that can be mixed and matched in the magazines, and each gun takes its own magazine. One error is that the Mauser pistol and carbine take Tokarev ammo (which will load in a Mauser but are too powerful to safely use, and require 7.63mm Mauser ammunition instead).

Simulation Game

 * Heavily, painfully averted in Operation Flashpoint and its Spiritual Successor Arm A. Unless the type of ammunition is used in a whole closely related family/series of firearms, you'll have to find appropriate ammo for each gun. You won't have much luck firing an AK-74 with a magazine of M16 bullets. As noted, however, related firearms can take the same ammo (most assault rifles of Western origin in the series use the same magazines as the M16, for example).

Survival Horror

 * Resident Evil 4. The Broken Butterfly is a revolver, whereas the Killer 7 takes magazines, which both use the same ammo. The ammo for the more basic handguns changes color to match the gun it's being loaded into.
 * Furthermore, the magazines are the same size for each gun, but the ammo cap for each gun can be upgraded. Is Leon putting two mags in on top of each other?
 * Though Justified Trope in the main game of Resident Evil 2 (Video Game) (both Leon and Claire's starting handguns do, in fact, use the same type of ammunition in real life), it is invoked poorly with Claire's bonus weapon, the Colt .45 revolver. Granted, it's to fit the whole "cowgirl" theme along with her outfit, but as its name suggests, the revolver uses .45 LC caliber rounds, not 9 mm rounds.
 * The first Parasite Eve game had this. The only unique ammo was the rocket launcher. Otherwise, you just had generic bullets. 9mm Parabellum = .45 ACP = 12 gauge = 7.62x51mm = 5.56x45mm = 40mm. For those who don't know ammo, pistol ammo = bigger pistol ammo = shotgun shell = rifle ammo = smaller rifle ammo = grenade.
 * Averted in Parasite Eve 2, each ammo type is Color Coded for Your Convenience - this even gets lampshaded by one of the NPCs, who notes that another character jammed a pistol by trying to force the ammo from a different pistol caliber into it.

Third-Person Shooter

 * In Dark Sector this is played pretty much straight with everything but the starting pistol; the Klin PP9 (9mm PMM), AKS-74U (5.45x39mm) and G36C (5.56x45mm) all share an ammo pool, as do the Webley Mark IV revolver (really a .38, but supposed to be a Nagant, firing 7.62x38mm rimmed) and the SOCOM 16 rifle (firing 7.62x51mm NATO).
 * Averted in Max Payne, where the only weapons that share ammo are the two 12 gauge shotguns. Every other ammo type is identified by the weapon it matches.

Turn-Based Strategy

 * In Jagged Alliance 2 all 7.62mm calibre weapons of WarPac/ChiCom origin use the same ammunition - 7.62mm, which would be logical, except there are three kinds of 7.62mm Warsaw Pact ammunition in real life: 7.62x54mmR, as used by the Dragunov and most of their MGs, 7.62x39mm as used by the AK47 / AKM and their relatives and 7.62x25 used by the PPSh/Type 65 and other pistol type weapons. They are definitely NOT interchangeable. This was eventually fixed in the v1.13 modification.

First-Person Shooter

 * Borderlands has over 17 million guns, with Universal Ammo for each type of gun. I don't think we need to explain this any further.
 * The Vanu Sovereignty in PlanetSide chamber the same type of ammunition (batteries) for their main weapons, as they all need similar levels of power. Their anti-tank laser cannon however, requires a much larger battery pack.
 * Deus Ex Invisible War takes this trope to the extreme. Anything that uses ammunition uses the same ammunition as everything else. The in-game explanation is that ammunition is reduced to a slurry of nanomachines that form into the proper ammunition for the weapon. This includes battery packs, flame thrower fuel, regular bullets, and even rockets. The creative reason behind this is that the inventory system got simplified, and carrying 20 types of ammo didn't sound appealing when you only had 7 spaces. This has the major drawback of burning your ammo with one gun means you're screwed.
 * Averted in the first game, and wasn't an issue because ammo was stored in a Hyperspace Arsenal separate from the normal Grid Inventory. That said, pistol and sniper ammo were simply cartons of loose bullets rather than magazines. That's One Bullet Clips for you.
 * Human Revolution averts this even further, as ammunition takes up inventory space and almost every weapon requires specific ammunition; only very similar weapons like the regular and double-barrel shotgun or regular and silenced sniper rifle use the same ammo. One Bullet Clips are still present.
 * The Blake Stone games have all the weapons using the same ammo. Justified in that all the weapons are energy-based and the ammo is energy packs. Just don't go too crazy with the more ammo-hungry guns that you have to resort to using your emergency self-charging pistol.
 * The Doom trilogy: the first two games had four types of ammunition for six guns (seven if you include the super shotgun). The Plasma Gun and BFG share ammo, which with them both being energy weapons powered by battery packs is justified. The pistol and chaingun also share ammunition, which is less justified. Doom 3 averts the trope entirely; even Plasma Gun and BFG now use separate ammo.
 * Wolfenstein 3D uses this trope, but to its credit, there's only three guns.
 * And the Luger pistol and MP40 SMG both use 9mm cartridges (though not from the same magazines.) The minigun? Well it is fictional.
 * Back to Half-Life: the gluon gun and tau cannon share the same (nuclear) battery packs. According to the Half-Life wiki, these weapons are powered by a revolutionary miniature nuclear reactor fueled with depleted uranium - the content of said battery packs.
 * Partially subverted in Dark Forces; despite all the weapons being energy-based, they use two different kinds of battery packs/cells. The main exceptions are the Dark Trooper assault cannon, which uses two different types of ammo exclusive to it, and the explosives, which are individual units.
 * In Star Trek: Voyager Elite Force Federation weapons use energy, while the those of the Delta Quadrant aliens run off dilithium crystals.
 * The Battlezone 1998 games' Hover Tanks feed their machine guns, cannons, rockets, mines, extras from mines to the Phantom VIR and even energy weapons from the same "nano-ammo" supply.
 * Return to Castle Wolfenstein uses this in two ways, first for the MP40 and Sten both using the 9mm Parabellum rounds (the British designed the Sten specifically to use German ammunition for work behind the lines) along with the M1911 Colt pistol and Thomson both using .45 rounds. The other guns, however, each have their own ammunition.

Platformers

 * Metroid Prime Hunters have actual nanotech-based Universal Ammo that fits all of your special weapons. Only the missile launcher uses different ammo. Since all the guns are at least partially energy weapons, it's a bit more believable, though, as the ammo itself probably doesn't need to shape itself to extreme tolerances - even for the explosive Battlehammer, "close enough" will do, allowing a quick configuration to be at least somewhat plausible.

Role-Playing Games

 * Mass Effect also takes this to the extreme. Ammunition in the game is nothing more than a metal block that gets shaved into the proper form. However, unlike above where it's crippling, this game has as-good-as-unlimited ammo, with rate of fire being limited by the weapon's heat sink. The block in a standard assault rifle is about 5,000 shots, which is more than enough to fight a war, and with field tools a new block can be swapped into place in a matter of minutes. In Wrex's back story, he once fought a duel that lasted for a number of days. He ran out of ammo and had to take a gun from a merc he'd killed in the process.
 * Mass Effect 2 kept the unlimited ammunition, but added "thermal clips" that absorb heat from the weapon and eventually need to be discarded. Essentially, they act just like ammunition. The clips are universal - pick one up and it adds more total shots to all of your weapons. However, the game does feature a more straight example of the trope with the Heavy Weapons, which do use traditional ammo... the same ammo, whether the weapon is a rocket launcher, an experimental BFG, or an alien energy weapon built from technology beyond what anyone else has. That this "ammo" is actually a high-density power cell doesn't explain either its universal compatibility or the weapons' ability to form complicated projectiles from that raw energy - though it may be that, like standard mass effect weapons, at least the second point is rendered moot by the weapon typically having more than enough ammunition to go through a full load of energy dozens of times, and being serviced on return to the Normandy. Also, Shepard can run out of thermal clips for one weapon, yet still have plenty of shots left in the others, which is inconsistent with said ammunition being universal.

Third-Person Shooter

 * This was earlier implemented to much less outrage upon reception in Oni. Oni has two kinds of ammunition: Kinetic, which supposedly consisted of explosives/propellant and slugs, usable for everything from SMGs to rocket launchers, which configured them appropriately on loading; and energy, which was for things like typical plasma blasters and tasers. The bigger the gun, the fewer shots any given ammo will provide before being used up.

Wide Open Sandbox

 * Grand Theft Auto San Andreas' pistols don't share ammunition (one fires a bigger bullet than the others, and another presumably uses lower-velocity ammo to complement its suppressor). The SMGs and shotguns can take ammo from other weapons in their class.
 * However, the AK-47 and M4 both share ammo.

Film

 * Most blasters in the Star Wars universe use a substance called Tibanna Gas and Power Packs. Apparently power packs come in a universal format too.
 * KoTOR 2 had you run a across a Proud Warrior Race Guy who was hiding from some predators. If you choose to mock him for managing to run out of ammo, he will ask if you have ever emptied a blaster and needed to reload.
 * Power packs are separate from the blaster's supply of energetic gas; an E11 as used by the stormtroopers requires the power pack to be swapped out much like a rifle magazine every 100 shots, while the gas will last for about 1000 shots but require more detailed work to refill.
 * The Dark Forces Saga had one type of blaster ammo in the first game, and two in later games, both of which could be recharged from the same power stations.

Literature

 * Similar to the Invisible War example in above, in John Scalzi's Old Mans War, the rifles used by the Colonial forces use nanotechnology-based ammo that reconfigures itself into whatever you're asking for--including rockets, grenades, bullets, and microwaves. You can even create your own custom firing macros (eg., fire a grenade followed by a blast of flame for good measure)

Live Action TV

 * Humorously averted in a scene in Burn Notice where Sam is needling Michael about having no life outside the spy business. Sam offhandedly mentions that his girlfriend Veronica once asked him if bullets came in different sizes.
 * Being forensics shows, CSI and its spin-offs and bandwagon-jumpers routinely avert the trope. In a notable recent example on CSI, the very fact that the rounds used in the crime were of an unusual caliber and design was a plot point. The rounds aren't even commercially available in the States. Turned out they came from.
 * Humorously averted in Shameless (US). Frank gets hold of an old WWII Luger and wants to shoot it in order to scare his girlfriend. However, there is no ammunition for the gun so he just grabs some random bullets that fit into the gun. When he tries to fire it, the gun blows up in his hand.

Tabletop Games

 * The tabletop RPG Shadowrun suggests handling ammo this way: all weapons of the same class use the same ammo, for simplicity.
 * Then again, its Sega Genesis game fits the above Inexplicable Example table, as Ares Universal clips not only fit any gun they're loaded into (regardless of class), but round themselves off to maintain a roughly equal percentage of ammo remaining across unequal magazine sizes.
 * Played fairly straight in BattleTech. No matter where or when in the competing Successor States the weapon and ammunition were manufactured, as long as their general type matches it'll fit; AC/5 ammo will work in any AC/5 ever built, never mind that those can canonically come in different calibers. The weapons and feeding systems are even flexible enough to handle special-purpose ammunition that is canonically sufficiently heavier than normal to halve the number of shots! Yet on the other hand, the 'general type' match must be strict -- no shooting standard shells from a more advanced (say, LB-X or ultra) autocannon of the same size or vice versa, no feeding missiles from an LRM-15 bin into an LRM-5 rack despite the fact that both get 120 identical missiles to the ton and feeding launchers of the same size from any matching magazine mounted somewhere on your unit is A-OK.
 * Painfully averted in GURPS: High Tech which gives an exhaustive list of ammunition types. Played more straight in Ultra-Tech where there are only a dozen different rounds between all the guns.
 * An odd version in the Wasteland RPG. Ammo calibers were standardized (.45, 9mm, 7.62x39mm, and energy), but the clips themselves were 'universal'--a rifle ammo clip could fit in any rifle. Their capacity was determined only by the weapon they were loaded in: the same clip that put 7 rounds in a .45 pistol gave 30 to a submachine gun.
 * Kult handles this absurdly, stating that any weapon can fire ammo of the proper caliber or smaller.

Toys

 * Nerf darts do fit with the majority of Nerf guns, it's when you have a non-Nerf brand gun that you run into problems...
 * Of course, darts come in a variety of diameters, which obviously won't fit smaller weapons.
 * Guns like the Recon and Longshot however can only take streamlined darts designed to work in their clips. These same streamlined darts will work in most other nerf guns, but not as well as the darts that are meant to be used with them.
 * Depends on which blaster though. Streamlines can be used effortlessly in the Maverick, requires some modification to be as accurate and have a similar range to other darts for the Barricade, and they will flat out not work in Muzzle Loaders like the Sharp Shot, Nite Finder, and Scout without modification.

Real Life

 * Kind of Truth in Television for armies of the world. The less kinds of ammo you have to stock and distribute, the easier it is to supply your troops. Imagine the logistics nightmare during World War II for the U.S., when any one squad needed .30-06 (M1 Garand and M1914 Machine gun), .45 APC (Thompson and M3 Grease Guns), or .30 carbine (M1 Carbine). Compared to today where a squad usually only needs one caliber: 5.56mm. 7.62 is used quite a bit, but this is more for designated marksmen and machine gunners carrying the M240.
 * The Colt Single Action Army Revolver or "Peacemaker" was available in .44-40 Winchester, which made it's ammunition interchangeable with the Winchester Model 1873 lever action rifle. A useful trait in the wild west.
 * Fabrique Nationale's P90 Personal Defense Weapon and Five-seveN pistol share the same ammo, the 5.7x28mm cartridge, expressly designed for both guns. The AR-57 goes one step further - it is an upper receiver for AR-15 rifles that can be used to make them fire 5.7mm bullets, even loading them from the exact same magazines designed for the P90.
 * This is the main reason for adoption of assault-rifle caliber "squad automatic weapons" versus battle-rifle caliber "light machine guns." Note that the latter were valid examples of this trope when battle rifles were general issue.
 * It used to be that machine guns and main battle rifles always shared the same caliber. There was just that period during the late 1960s to the late 1980s that they were different, at least in the US. But it also makes more sense to have the squad-assault weapon be of smaller caliber. Those weapons are designed just to spray lead downrange to keep the enemy pinned. Smaller ammunition means more rounds for the same weight. Thus, they last longer.
 * In general, submachine guns are issued in the service pistol caliber of the nation in question.
 * One of the major reasons for the controversial U.S. Army replacement of the Colt M1911 pistol with the Beretta M9 was so that NATO armed forces would only need to stock one type of pistol-caliber ammunition (the M1911 was chambered for .45 while pretty much everyone else was using sidearms chambered for 9mm).
 * One of NATO's standardization agreements (4179 specifically) decreed that all NATO weapons chambered for 5.56x45mm were to be designed to take the same magazines as the M16.
 * Generally, while there are different kinds of calibers, many shotguns tend to take 12 gauge shells.
 * Similarly, as with above, pistols chambered for 9x19mm are by far the most common.
 * As of 2012, the main service pistols of the world's three largest armies fire the 9mm Parabellum. US's M9, China's QSZ-92-9(5.8mm variant was not adopted), and Russia's MP-443 Grach. With the majority of both NATO and SCO's armies using the 9mm, it has become the universal pistol caliber.
 * One of the main advantages to the .22 Long and its variations was that a rifle fitted for one caliber could take any that had a shorter casing. A few other calibers (the .38 for instance) also did this. This only works with rimmed cartridges though.
 * A revolver chambered for .357 Magnum can safely use .38 Special or +P rounds. They're the same diameter, but the .357 case is slightly longer, so it cannot be used in a .38 weapon for safety reasons. Likewise, the .44 Magnum and .44 Special.
 * This sort of thinking was the reason why nearly all Soviet small arms from at least 1891 to about 1974 fired 7.62mm bullets - even though they had different such bullets, if they ran critically short of barrels for anything during wartime production, they could take old Tsarist-era Mosin-Nagant rifles and recycle their barrels for use in other guns.