Forget the great unwashed, the poor, the benighted.

Forget money sucking mercanaries from Blackwater

Think about the pissed off ex-marines and soldiers.

The legionaires who came home, avoided the mutilation of IEDs, and know how to 'clean and cock', and can lay hands on weapons.

Think Russia 1918, Germany 1921.

Think Spartans and Friekorps.

Type Friekorps into Google or Wiki

Think pissed off, let down, left in the lurch, stabbed in the back.

The USA has the largest number of ex military types in the western world. You cycle more young men and women through the armed forces than any other Western nation. Assume that they did not forget the training when de-mobbed. Assume that going through hell+ returning to a shit job+ subprime mortgage may be considered a bit of a wind-up

All can get legal access to weapons.

Disenfrachised Ex military types + over the counter weapons = civil war.

Last time these conditions occured in Britain, we chopped the head off a King.