To be fair, all of those weapons you mention were only in Fallout: Tactics, which is of semi-dubious canonicity. Also, the FO3 Chinese AR is more likely the Norinco Type 56 with its wooden furniture rotting away.
Doesn't matter. The Pancor Jackhammer is lore friendly as it was in Fallout 2. The rest were deisgned and, except for the Kalashnakov, built prior to or during WW2. We know WW2 is
before the Fallout timeline diverged, thusly anything that existed prior to 1945 can be modded into a Fallout game without upsetting lore at all. The M2 Browning was in Fallout 1 and 2, and the Chauchet was a joke big gun in Fallout 2 that worked about as well as the real one did. We also know the AK47 existed in some form or another in the Fallout timeline because an AK112 does exist.
Know what I wanna see?
A Fully-Automatic Shotgun.
Specifically, the Pancor Jackhammer. Yeah, it was developed in 1987, but it never entered production. The only problem I can see is that it's design may be a bit too "futuristic" for the Fallout Series (when I say that, I mean Futuristic as how we envision it, not how the 1950's envisioned it.)
http://en.wikipedia.org/wiki/Pancor_Jackhammer for reference.
It was in Fallout 2 right from the getgo, so it's lore friendly. It has been added to Fallout 3 by the community.
You'll see a Pancor Jackhammer in New Vegas sooner or later.