I'd be perfectly okay with local PD swat units (multiple... for a total of about 50 officers or more) setting up fixed position LMG emplacements.
8 SAW's around the perimeter, with roving patrols armed with M4's and a 2 dozen man rapid response team on-site would make short work of a violent mob of 500 people, as long as they were unarmed.
Just on general principals, I think they need to arm the security guards and tell them that I'd understand if they were in fear of their life if anyone tried to break down the gate. Next time, the police should respond to several mags worth of bodies outside the grain terminal fenceline. If were' going to pretend to be civilized, we can't allow the uncivilized to dictate our behavior.
However, if they are already at the point of property destruction... I expect many of them are already covertly armed with handguns. An overt show of force with LMG emplacements and swat units will probably result in the next mob, if they choose to rise to the challenge, will be armed with long guns. 500 poorly trained folks with various long guns will most likely defeat a 50 man force with LMG and automatic weapons. Depends on how passionate the attackers are for their cause, or how much the defenders want to defend.