There are a few AK47 mods, here is one of them
http://www.nexusmods.com/newvegas/mods/48392/?
There are also mods that add other weapons and include AK-47s
The thing is the game is based around the 1950's cold war vibe and commies are bad and since AK's are associated with commies and no law abiding American would associate with using a commie gun. There was the McCarthy Hearings back then as well to weed out Commies in America.
If the game had you facing against commie forces than yes I would expect to see AK's and other communist weapons.
The M-16 and M4 would be more common since they are are American based and would be made in the States. AK's would not really be made in the states especially if the game world is based on the 1950's cold war vibe that had a nuclear war.
The Chinese assault rifle in F3 is kinda the AK in the game. I too wish they used real life weapons PLUS fictional weapons that they created.
This.
http://fallout.wikia.com/wiki/Chinese_assault_rifle, during the Gomorrah quest line.
Capitalist propaganda! You can throw an AK-47 into the mud and it would still be functional; its simplicity and reliability is legendary. In the lore, the Chinese Assault Rifles are usually smuggled into the US for Chinese sympathizers and operatives, and they remain functional after 200 years. So I assume that all the CARs in Fallout 3 are 200+ years old.
The AK-47 is in Fallout Tactics for what it's worth.
In the Orginal Fallouts there is the http://fallout.wikia.com/wiki/Assault_rifle_(Fallout) So I would guess that by the time of the Great War, the AK-47 was replaced.
I see no reason why there wouldn't have been an American version of the AK-47. There are plently now and we know Fallout Universe had a gun that is based on it called the AK-112. It would be nice to have an AK-47 looking weapon that wasn't the Chinese Assault Rifle.
The AK-47 would have existed in the Fallout Universe. But we also have to remember that America wouldn't have been importing them because they are commie weapons!
Also don't for get about the http://en.wikipedia.org/wiki/StG_44 I could be wrong but it seems to be the AK-47 is just a cheaper mass produced version of it.