Of course what is legal depends upon the immediate circumstances and the reasonableness of one's actions. The bad news is that often you'll be in court to find out what folks in your community see as 'reasonable.'

An example: There was a coal mine strike in NW New Mexico. Strikers were encamped by where the mine road met a public highway. The coal company continued operations using manasgement personnel (even as oilers on the dragline - their suits got messy) and a few 'scabs,' or workers crossing the picket line. All vehicles had to stop where the mine access met the highway (both because of the law and heavy traffic) - strikers surrounded a 'scab's' car and started beating it and screaming at the guy inside. When someone started using a chain on his windshield, he pulled his 303 Enfield off the floor, fired one round out of his driver's side window, nearly straight up. The crowd backed off posthaste.

The DA's analysis was that his use of a warning shot was close to reasonable under the circumstasnces and was not something they would prosecute. A different DA might have decided differently.