In the early fallout games, there was a fn-fal rifle. At the time I knew next to nothing about guns beyond Dirty Harry's .44, so I think I thought it was fictional.
But of course it's not, it's the real world Nato weapon, their counterpart to the US choice of the m16, apparently.
But there was nothing like an m16 in fallout 2. Interesting, then, that "Nato" weapons were more common than the m16. It has me believing that the US went with the Fn-Fal like the Europeans, and the m16 just never happened (Until its cousin, the NCR service rifle, I guess.)
Does that sound right?

IIRC the m16 appeared in Fallout: Tactics.

I speculate that America, during the Fallout time period, had a large diversity of guns both local and foreign(AK112, HKG11E, FN FAL, Steyr Aug, M16, Ruger AC556, R series assault rifle to name a few) with no restrictions on ownership.

There's no m16 in fallout 1 through 4, as far as I know. It feels like that's an unusual choice to leave out, when they had some much more obscure weapons of foreign origin that are quite common. Alternate history is all I can assume, because the m16 is a weird weapon to accidentally forget about.

The FAL was more of a European counterpart to the M14, not M16. As for why the M16 isn't present in any Fallout games other than NV, maybe it's just too modern looking for the Fallout aesthetic?

The g11 is more futuristic and obscure than the m16, but it had a good presence in the game. Considering it was only pretty much a prototype in real life, I guess that's another bit of alternate reality in action.

The FAL was almost adopted by the U.S. after WW2. While European NATO countries adopted it, U.S. tested it against several upgraded variants of the M1 Garand. Because of those test, the M14 was developed and used as the U.S. standard battle rifle. So it’s too much of a stretch of reality having the FAL vs the M16. I think it’s interesting how Fallout 3 randomly brings in the PTR91/HKG3 into the Fallout universe, when in reality they were
banned in the U.S. and the only adoption was from Law Enforcement, and the majority of HK adopted firearms were MP5’s.

Fallout is a divergent timeline. This divergence occurred in the mid 50s. The FAL was developed in 54, and the M16 10 years later in 1964, post divergence. The only iteration of the weapon in game occurs in tactics, which contains many things that heavily contradict the canon and established tech of the world (Brotherhood producing power armour, modern military vehicles).

It's important to note that the Service Rifle we see in game is NOT an M16. It's design is based off this real world weapon in order to fit in with the theme of the NCR as an emergent organised military force. The weapon is very noticeable for it's vietnam era imagery, much like the uniforms are akin to WW1 fatigues, it's to create a theme of an army both able to mobilise and equip in mass. The weapon we see in game is for the most part, a post war gun. They are produced, most likely by the Gun Runners based on shipping manifestos found in game, with cost efficiency and combat efficiency balanced by design in order to equip a large force with an effective firearm, hence the name, Service Rifle. The only pre-war iteration of this weapon is the Survivalist's Rifle, which was produced by a Canadian company called Long Branch Arsenal, which actually did have a real world counterpart, though that ceased operations in the 70's. It was likely some of these weapons were appropriated by the US military, but these weapons chambered 12.7mm, and is a carbine, making a great distinction between the two rifles. The service rifle is for all purpose, a post war weapon produced for the NCR's army, not the US military, unless stated otherwise.

The R91 was the standard issue rifle of the US by 2077. While it may look like a different real world weapon, it was a rifle manufactured in the US for the US military. Those similarities, and any other example, unless otherwise stated, are purely down to artistic inspiration on the part of the concept and design artists, and as such they are not considered the same weapon. Fallout is a divergent timeline. While some companies and items do exist in both of our worlds, these primarily existed pre-divergence, such as the FAL and the iterations we discover in game are extensions or improvements of the weapons that existed back then, much like the Chinese AR's similarity to the AK, or its pistol to the Mauser.

@ahyuser001
America pre-war was not entirely unrestricted when it came to gun/armaments ownership. The Bureau of Alcohol, Drugs, Tobacco, Firearms and Lasers was updated from its real world counterpart, (Hence the addition of Lasers) and was known to restrict ownership of things such as Combat armour and the DCTA laser firearms protocol found in Fallout 3 shows that paranoia in regards to events such as communist invasion did mean that many people, including subway maintenance officials were armed, though under other strict rules, like not using it to kill pests, and keeping it under log. Generally, protection against foreign threats was the main reason for this, and it's likely many companies took advantage of that, hence the wide assortment of Firearms and ammo found across the wastes. Also, the House Tools Company shows us that this paranoia also extended to things like machine gun/laser turret placements in office spaces, and other businesses like LOB (a Chinese front) actively armed their workers in order to combat a possible federal invasion. America pre-war was incredibly capitalist society. It was built entirely around commerce and business, hell, if Fallout 4's intro is anything to go by, even the Fat Man could be purchased, which it seems did occur from the amount of Mini-Nukes and Launchers we can find in pre-war buildings.

@"IRORIEH"
Damn forgot about the BADTFL.

Someone already mentioned the U.S. almost adopted the FAL so there is that. I mean maybe they left out some real life U.S. weapons and chose their euro counterparts because in the 90's people wouldn't have been familiar with those types of weapons. It's the same reason in a lot of like futuristic or post apocalyptic movies from the 80's they use unfamiliar weapons to pass off as 'futuristic'. The AR-10 and FAL were developed around the same time
@IRORIEH
I mean Bethesdas lore is extra contradicting ya know what i'm saying. They say r91 but i really hear AER9. Then there was that other weapon mentioned in fo3 the 'Backtalker'. I always assumed that was there version of the m16. We know they had to have had AR/M16 variants of weapons in service though. The All-American, Survivalists rifle, Assault carbines, marksman carbines, service rifles. Maybe because the way the states are broken up into 13 territories they decide how their National Guard units are armed. Maybe i'm thinking too much about this. Sorry to disturb you.