I think the M16 looking one may be something that the NCR produces or its a M199 Backtalker. The R91 was its replacement so its possible that the military units in the area had not yet made the conversion over.
That's true, IIRC the R91 was the main combat rifle of the army at the time. The reason it was so prevalent in DC was because Martial Law had been declared and there was a massive national guard presence in the area, who were all equipped with frontline quality weaponry in anticipation of major riots.
Other areas might have older, less standardized weaponry in their arsenals, like the AK-112, the Backtalker, or whatever these guns are.
It would be cool to see the AK-112 come back, especially with some more backstory.