I know I'm probably nit-picking so it's not a big deal.
Coming from fallout 1 and 2, I sort of wish that the NCR service rifle was based off of the fn-fal (or whatever the lore says the old US used as a service rifle.)
I assume that the ncr were armed with something that looks very much (to my non-gun owning eyes) like an m16. Probably to make it look like the ncr is sort of mimicking the old world. Except that the old USA probably didn't use the m16 as their pre-laser firearm of choice. It didn't even exist in fallout 1 and 2 if I recall, which means it probably was not widely adopted, at least not in the lore as it stood at the time.
I seem to remember reading that in real life the fn-fal almost got adopted as the rifle of choice but finally was beaten out by the m16. The common-ness of the fn-fal seems to indicate that maybe it won out in the fallout universe.
Until later, when this seemed to be ignored in later games, I guess.

If we break it down:

The original Assault Rifle in 1&2 was the "AK-112" which bears a striking resemblance to the "Colt Rangemaster" which is a "Marksman's/Hunting Rifle" but with a hooded front sight and a larger magazine

Now, in regards to the M199, we only have a snippet of info from Fallout 3.

Speaking of Fallout 3, that was the one that introduced the H&K G3 looking R91 Assault Rifle, which didn't see any use in Fallout New Vegas, but did see usage as a sign in Fallout 4, but according to the lore was a Nat Guard weapon, not a US Army weapon.

But Fallout 2 also had the H&K G11 K1 and a DMR version, as well as an early version of what became the SA80 and the FN FAL as you mentioned.

Fallout Tactics had the M16A1 in it, and the AK-47.

It seems like there's absolutely no consistency in the lore with regard to assault rifles. It seems strange, that if the US army was using the m16, that they would appear only in one game. In fact, any apparently common weapon that appears in only one game seems strange.

Let's see how many Assault Rifle, Carbine and Designated Marksman's Rifles are in current US service (note, I've mixed them as often some are just one with full auto removed, and the games mix them up a bit)

M16A2
M27 Infantry Automatic Rifle (HK416 - USMC)
FN SCAR (L&H models - SOCOM)
Mk 14 Enhanced Battle Rifle
M14 rifle (Sealift Command)
HK416 ("Sneaky stabby" groups)
M39 Enhanced Marksman Rifle (M14/Mk14 modification - USMC)

So yeah, plus the original lore didn't mention about anything the usage of the M16, probably because they didn't want to tie it down too much, plus legal issues, look at how they have fake name brands for stuff (Nuka-Cola instead of Coca-Cola) so didn't go into it much.

As for the lore about why the US Nat Guard where using the R91 and the US had "standardised on the R series to replace the M series" was from Fallout 3, but we don't know what that change was exactly, or how close the Fallout timeline stayed to ours before diverging off harder.

Hell, this discussion so far has only been about 1 country's worth of weapons, when we start mixing everything else in ... we end up going off into massive numbers of "Current, emergency reserve and long term storage" weapons.

Plus, how long does an M16 survive without any real maintenance, as I doubt everyone would have access to non-corrosive ammo and the stuff to properly maintain and clean an M16 for 100 years, let alone 200 years, especially something with as many fine moving parts as that has.

So... according to the seemingly inconsistent lore, what would be the NCR service rifle, if in fact they were attempting to imitate the pre-war military? I'm still skeptical that it would be based off of the m16, despite it showing up in tactics.

@Sirdanest

I think the lore inconsistency is from the difference of perspective from Bethesda Softworks and Brian Fargo. Bethesda's take on the lore is that in Fallout, America never got out of the 50's culture wise which is why they have nuclear future-esque tech.

Brian Fargo's take on this is transistors were never invented in the Fallout universe which is why tech was so big because of the vacuum tubes those were using.

So while you see weapons developed after the 50's in the original games, it will change when you play the Bethesda Fallout games.

To the question on the FN-FAL, if you think about it the weapons the NCR had during fallout 2 was the assault rifle/AK112 which as McKaby stated looks similar to the colt range master/AR series of assault rifles.

It would not be weird if the Gun Runners somehow developed a replica of the M16 and turned it into the NCR's standard issue assault rifle using the assault rifle/AK112 as their basis for this weapon's development.

I wish to point out the Transistor not being designed was a fan edit to the Wiki, it's often discussed on the Fallout Lore subreddit, so I question that (if you have a source, please share it)

But I feel that Fargo's iteration was that America peaked in the 70/80s and then went into the "lets rehash 50's Americana and go from there" which I feel would have been a better fit.

Also, the Colt Rangemaster isn't a real gun from what I can tell, but like I said, it and the AK-112 seem to be the same weapon with minor differences.

Though it seems the AK-112 was called the "Colt Rifle" and the Rangemaster was "Colt Semi-Auto Rifle" in early development, so that shows a link as well.

But yes, the FN-FAL was only in Fallout 2, but from what I can tell, wasn't the standard NCR rifle at all.

I don't think the Gun Runners morphed the AK-112 into the M16, as it seems too much of a big step, but the M16 could have been an old weapon that they found the plans for and made into the NCR's standard rifle through lobbying (which we know the NCR has a big problem with.)

Honestly, I just put it down to that there where many a rifle that the NCR used until they standarised on the M16, heck, they might have been standardising on the AK-112 before being shown the plans for the M16 that the Gun Runners had, or maybe even a working one.

The fal is actually far more sensitive than the AR15 platform is. The Israelis actually dumped them (and their galils) for m16a1s at some point. And they were fighting in the desert as well. The M16 is relatively easy to maintain and replace parts and due to its sealed action and receiver design it's extremely reliable in adverse conditions. Even more so than the Kalashnikov platform (contrary to popular belief as well as communist propaganda).

Makes sense why the NCR would use it.

A surviving blueprint pr functional copy for the m16 must not have been in the gun runner's original supply, but apparently one probably did exist in tactic's part of the country, and eventually that spread back west to the gun runners. The 10mm pistol has often been described as durable and something that could have survived, and it's pretty common. Weirder stuff like the p90 and hk g11 were probably only in the game "because they look cool/different" but can easily be passed off as gun-runner experiments.

I mean, I wish I didn't have to write my own fan-fiction almost, to explain the lore, but at least it's plausible.

I don't keep track of real life gun lore too closely but I think there's supposed to be a shotgun that has very little recoil and can be tossed in a lake, fished out, and fired like new. So some guns clearly can take a beating, I'm just not sure Brian Fargo or Bethesda had any reason for including any weapon other than that they just wanted that weapon in the game.

Pretty much that and "can we legally do this without getting sued" as that's happened before with one of the FPS' where the company was sued for using a company's name and product without having had a legal agreement in place.

But yeah, there isn't much lore out there, same as why we got that weirdass LMG becoming the assault rifle, the only reason is that apparently Bethesda decided that having it and the Chinese AR resulted in the C AR being ... redundant.