In the 1970s, a civil war erupts in the US, maybe trigerred by some racial unrest.
The war gets really bloody, the US government falls apart.
The war lords replacing the government are happily supplied by the Russians with AK47s, while the Russians take many of the scientists, engineers, and so on to Russia, where the M-16 soon becomes the standard weapon for the army.
A fundamentalist Christian regime gets dominant in the US, after most of the Infrastructure is destroyed. Both their religion and ongoing resistance by rebels keeps the US from producing their own weapons, so that the AK47 stays standard weapon of the new US.
Edit: I overlooked the part with "nothing else changes"...