I have no problem with the military and police having access to weapons. They are (or should be) highly trained, Psych tested and they work within a regulated, highly structured, hierarchical organisation. All of this serves to keep the wanton violence (almost completely) in check.

The issue arises when every schmoe off the street can go in and purchase a fully automatic, purpose built people killing machine.

I also don't buy the people vs. the government scenario you allude to. Could you really picture a reality where a group of citizens could overthrow the US government just because they had some AK47s?

In the event of run-away government, a far more rational and realistic scenario would be for mass protests that essentially halt the economy over night. There are countless examples of almost completely bloodless revolutions happening in exactly this manner (Polish Solidarnosc and more recently Egypt during the Arab Spring immediately come to mind). The alternative is a bloody coup which usually ends in civil war or military dictatorship.