Well, there have been instances of the U.S. Military being used against civilians:
1) The Bonus Army
2) Kent State
3) Katrina
4) Waco
I wouldnt bet my life that the military would stand down. I think it would be a mixed bag if the order was given. Some will follow orders, and some will rebel.
The National Guard was used in the first L.A. Riots. Watts, I believe that it was. The National Guard was also called many times in labor disputes between management and labor involving coal miners for one. Back in the ‘20s and probably in the early 1900s.