re: when am I allowed to use/brandish a gun?

1. Brandishing (called "illegal carry" in Louisiana statutes) is illegal. Drawing a handgun in self defense isn't brandishing.

2. As was said, driving to your home after a confrontation isn't wise. Drive to a police station.

3. ALWAYS phone in a threat to the cops -- unless you're the one who did something stupid to instigate it. The failure to phone in the incident makes me doubt the story told by the OP's buddy.

4. In Louisiana you're allowed to use deadly force when "in reasonable fear of death or great bodily harm." Having someone follow you doesn't necessarily rise to this level. Nor does someone yelling at you from a car.

If you shoot someone under the circumstances described, you better be able to do a good job of convincing the DA and/or a jury that the threat was sufficient to meet the legal threshold. Just being "scared" isn't how the law is worded.

5.HOWEVER, if someone tries to force their way into your car while you're in the vehicle, or illegally enter your home while you're in it, you'd be on firmer ground. But even then your use of deadly force has to be "reasonable."