Selective Historical Armoury

Military fiction set in the past, World War II in particular, will often inexplicably leave out weapons that should be very common, often in favour of "cooler" but rarer or even anachronistic items. In films, this can be explained by availiability of props, but video games have little excuse other than the developers cutting corners, especially if they took the time to model other weapons that have little in-game use, like pistols.

The opposite of Rare Guns: In these cases, it is weapons that should be abundant in the setting that are conspicuous by their absence. Compare with Anachronism Stew, Video Game Historical Revisionism, Improperly Placed Firearms and Weapons Kitchen Sink.

General

 * In any military fiction set after 1960, generally only basic Soviet weapons like the AK-47 and RPG-7 and common American types like the M16 rifle, M60 machine gun and M72 rocket launcher will be seen. AK variants and derivatives are far more rare than in real life, and extremely common rifles like the G3 and FN FAL are seldom seen.

Film

 * The 2006 film Flyboys, set in WWI, featured the Fokker Dr.I triplane exclusively as the fighter plane of the German air army when it was in fact not terribly common. More egregiously nearly every one is painted bright red, when the only pilot to use an all-red scheme was Manfred von Richthofen (hence 'The Red Baron'), and even partial red paintjobs were generally a trademark of Richthofen's Jasta 2.
 * Also, several of the French and other Allied planes shown are anachronistic, because they were either non-existent or phased out during the events of that movie (it's supposed to be set in 1916).

Literature

 * Subverted in Harry Turtledove's Alternate History novel The Guns of the South, where the Confederate army use AK-47s because they've been imported by time-travelling white supremacists who want the CSA to win the American Civil War, so slavery can continue.
 * An almost identical plot appears in Harry Harrison's short story A Rebel in Time, written nearly a decade earlier. The only major difference is the weapon being smuggled - a WWII British Sten - which would arguably be an even better choice, since due to its famous simplicity, it could easily be manufactured even with basic 19th century industry (pretty much at a village smithy).

WWII First Person Shooters

 * World War II games in general often forget to include the M3 "Grease Gun" when it comes to American forces after 1943. Created as a cheaper replacement for the popular M1 Thompson submachine gun, it saw widespread use for many American units in Europe around late 1944. Despite this many games will either leave it out because the designers feel it's too similar to the M1 Thompson, or will make it an incredibly rare gun that will only be seen a few times in the game. (As seen in Brothers in Arms: Hell's Highway and Call of Duty 2).
 * Games that feature British player characters seldom offer them the Thompson either, despite it seeing quite a bit of service alongside the more common - if persnickety - Sten.
 * Many games set in the Pacific theatre give Japanese soldiers German weapons instead. Ditto for the Italians in games involving the Mediterranean theatre.
 * Call of Duty has the FG-42 as a usable weapon, of which less than 7000 were made, while the American Grease Gun (which was very common in comparison) is conspicuously absent. Also, the only pistols available are the American M1911 and the German P'08.
 * Call of Duty 2: When coming up against German armoured vehicles, you must either run up to them and attach a sticky bomb, or in some levels find a Panzershreck reusable rocket launcher [fewer than 300,000 made] lying about. You and your British or American AI teammates will never have PIATs or M9 "Bazookas" available, nor will the opposition ever have any disposable Panzerfaust rockets [more than 6 million made] lying about. Though the developers did not bother to model these weapons, they did make the effort to model Enfield revolvers and several pistols, weapons that are rather useless in-game. In the first Call of Duty the Panzerfaust was a common sight, but so many complained that it was a single use weapon and rather had the Panzerschreck instead. On the other side, the M1 carbine is far more rare than it was in the first game, and while the M3 Grease Gun is actually included this time it still is nowhere to be found in singleplayer.
 * Call of Duty: World at War includes the PTRS-41 rifle (presumably as a 'historic' equivalent of the Barrett M82 sniper rifle from the previous game), even though the PTRS-41 is a massive caliber anti-tank rifle not at all suitable for being carried around and fired from the hip, as it is in the game. The first Call of Duty used it properly, though, as a stationary anti-tank rifle. Though against German Panzers, from the front (which didn't work).
 * In the Pacific Theatre of World War II, the Japanese mostly carried Arisaka bolt-action rifles; the Type 100, their only SMG, was rather rare, and production only amounted to a little over 10,000. Not so in Call of Duty: World At War, in which seemingly every other Japanese soldier packs a Type 100. The amount of man-portable automatic weaponry in the game is overdone in general.
 * Battlefield 1942 completely ignored how its weapons were used in actual history. The assault class of each army gets a historical machine gun which functions in-game as an assault rifle, and the engineer classes get the bolt-action rifles that were really the standard-issue weapons of most armies. The worst offenders, though, are the stationary machine guns, which do not cause a whole lot of damage, and whoever uses them stands up straight, completely exposed to enemy fire. Interestingly, the mod Forgotten Hope added historical weapons, and the result was a game which was much more authentic and more fun to play.
 * The Japanese in the game use German weapons like the Kar 98, MP-18, and Walther P38, as well as the experimental Type 5 rifle; Russians use American and British weapons, but have their own DP-1928 LMG as their main Assault weapon. The Japanese Arisaka Type 38 and Type 99 are seen in the hands of almost every Japanese soldiers. Oddly, the Russians uses the MP-18 instead of the extremely common PPSs-41. The British have Thompsons and BARs instead of Stens and Brens. Like above, the Grease Gun, which should be extremely common, is missing despite some of the battles taking place after 1943.
 * Not to mention that when playing as the Germans, you have access to the StG-44 before 1944.
 * The Russians with MP-18s are likely a balance issue, since the 71-round drum of the PPSh-41 would be unbalancing. Yet, the PPSh-41, as well as the later PPS-43, could also use 35-round box magazines (as seen here).
 * Battlefield 1943 continues the tradition with Thompsons being the only submachine gun available for the Americans; the Type 100 being the only submachine gun available for the Japanese; the German Karabiner 98 rifle being the sniper rifles for the Japanese; and the experimental, never-entering-service Type 5 being the semiautomatic rifle for the Japanese. As the game's options are quite limited in general and strictly multiplayer and all weapon types being exactly the same for both sides, though, it was clear they were just looking for a simple justification for the game to be easily balanced.

WWII Stealth Based Games

 * Commandos 2: Men of Courage features British commandos which standard weaponry is made of Lee-Enfield rifles (British), M1911 handguns (American), M1903 Springfield sniper rifle (American), PIAT (British) and Mills bombs (British). The German weapons available are K98K Mauser rifles (both scoped and non-scoped version), MP40 submachineguns and P08 Luger handguns. All good and acceptable, but several other common weapons like the Sten, Thompson, M3, Webley, and Walther P38 don't appear at all.
 * The first Hidden & Dangerous had Panzerfausts but no Panzershrecks, your SAS troops could outfit themselves with M9s, but not with PIATs. There were also some other odd absences, like the Colt 1911 being the only handgun available, and nearly all rifles only being available with scopes mounted. The expansion pack introduced a better selection of rifles with iron sights and the Luger P-08... But not its replacement the Walther P-38. Also, the player can meet some Soviet soldiers in the last mission of the first game, who carry the American carbines, which are the standard SAS rifle.
 * Hidden & Dangerous 2was a bit better in this regard. It had a reasonable selection of British, American, Japanese and even Soviet firearms was available, though there were no Italian weapons modeled, even though you fought them in a couple of scenarios. Anti-tank was still limited to Bazookas and Panzerfausts. The absence of the PIAT could be justified on the grounds that it was considerably more difficult and time-consuming to reload than the other two; Special Forces could and did requisition something rather better.

WWII Strategy Games

 * Close Combat II contains no jeeps or universal carriers, despite these being quite common in the historical operation. There was a mod that made the armed Jeeps used by the British paratroopers at Arnhem available. They turned out to be a Game Breaker for the Allies.
 * Come to think of it, the Universal Carrier (better known as the Bren carrier and best descibed as a cross between a Jeep and an APC) rarely if ever gets a look-in, despite being made in huge numbers for a variety of roles.
 * Battlestations: Midway (set during the period between Pearl Harbor and the Battle of Midway), features PT-109s during the Battle of the Philippines in 1941, despite the fact that the PT-109 wasn't even built until mid-1942.
 * Mostly averted all the way in the Steel Panthers series (and its improved Game Mods), since these titles try to be a realistic turn-based simulation of WWII combat.

Other First Person Shooters

 * This trope example is unbelievably averted, of all places, in Fallout 2. Mobsters in New Reno usually carry either Tommy Guns or Grease Guns, and only those two weapons use (quite heavy) .45 caliber bullets. There's even a blurb about how these guns ended up in National Guard armories after the Army switched to better weapons in its information screen.
 * Battlefield Bad Company makes this a natural part of weapon choice, and the sequel did it even MORE delibrately! A player could use the M1 Garand rifle, M1911 pistol and Thompson submachinegun, all explicitly having the label "WWII" in front of the gun's name when the message showing a kill with it and the weapon that did it.
 * Zig-zagged by Far Cry 2, where the assault rifles include the FAL and G3 next to the AK-47. The shotguns on the other hand go from generic pump action to the SPAS-12 and USAS-12. One unrealistic and "gamey" thing is that, due to the need to "progress" and gradually get better gear, the G3 does less damage than the FAL despite using the same ammunition and overall quite similar construction.
 * Rather than an M16 in the game, Far Cry 2 includes an AR-16. The AR-16 was a prototype version of the AR-18 Bushmaster assault rifle chambered for 7.62mm NATO rounds...making it more of an example of a Rare Gun. The idea seems to have been to have all four automatic rifles be 7.62mm rifles, but that ignores that the AK's 7.62mm round isn't anything like the one used by the other three anyway.
 * Somewhat averted in Call of Duty 4, where we see a number of AK variants, the G3, and several variations on the M4 carbine. However the AK-74 is only present in its AKS-74U carbine form, with even the Russian Loyalists using archaic AK-47s instead of the AK-74 which should be their standard rifle.
 * There's also no AKM or Type 56, both of which are far more common than the old AK-47, and the 'AK-74u' seems to have been modeled after some Yugoslavian or Bulgarian weapon rather than an actual AKS-74U.
 * Also played straight with a number of other questionable weapon/user pairings. For example, alongside their obsolete AKs the Russian Loyalists use the H&K G36C, while the U.S. military issue Beretta M9 (actually a 92SB) is the standard sidearm for all factions. To be fair, the Beretta 92FS (the US military designation is the M9) is a very common gun worldwide.
 * Modern Warfare 2 takes the odd weapon selections to the extreme, with the Brazilian militia using everything from lever-action Winchester 1887s, locally produced FN FAL's, to updated AK-47s with fancy optics, and the Russians using many foreign weapon designs, some of which originated from countries they would traditionally have nothing to do with, as is the case with the Israeli Tavor TAR-21 and French FAMAS. Basically, they use everything but what you'd expect.