I'd be interested in your LEO perspective on this.
Drawing upon Georgia law, which has no such brandishing law we have what is called pointing a pistol at another, which has specific exemptions for someone being in fear of great bodily harm or ones life.
I’d calm the situation down, separate both parties, ask Tom to secure his gun ASAP or I might even secure it, it would all depend on the atmosphere at the scene.
Interview as many witnesses as I could find.
If the stories added up to more or less what you described, I’d lay out the options.
A mutual release from prosecution, (Lack of Prosecution Form) or LOP, signed by both parties where neither side wants to press charges against the other, or both get hauled to jail.
I would explain to Bob that he has committed the offenses of simple assault and simple battery,
Simple Assualt which is defined as an act which places another in reasonable apprehension of immediate bodily injury. A mere threat of harm is insufficient to support an assault charge. A threat combined with the apparent present ability of a defendant to actually carry out the threat is necessary. The victim must reasonably believe that he is in danger of receiving immediate bodily injury.
Simple Battery occurs when the defendant intentionally makes physical contact of an insulting or provoking nature or intentionally causes physical harm.
As for Tom, what he did would be open for the judge or a jury to decide, here’s the law for pointing a pistol at another.
Title 16, Chapter 11, Section 102 (16-11-102)
A person is guilty of a misdemeanor when he intentionally and without legal justification points or aims a gun or pistol at another, whether the gun or pistol is loaded or unloaded.
Personally, Tom was justified IMHO, but based on my experience I guarantee you dinner anywhere you want if I told Bob that he was going to jail to facing more charges than Tom, unless he signed the LOP, he would sign it.
By doing this, I am covered, the department is covered and you as the person who cleared leather you are covered because now, I have obtained in writing for me and you the statement from the person you had at gun point his desire not to press charges against you, so later if he did come back and want charges the DA would grill him or her real good over the change of heart and most likely after all facts are known and they talk to me would not allow Bobs charges to move forward because any good defense lawyer would use his written statements in court to impeach him, not withstanding the fact he entered a false statement in writing.
Which in Georgia is a felony, but in this case I doubt it would get pushed although I’m sure the defense lawyer would scare the ???? out of Tom with the threat of further charges.
Oh yeah, I guess Tom would tote the citations for the accident, since he was indeed at fault by your description.
Direct means there is no way to withdraw or other physical protection such as a door or barrier between you and the assailant. Overt means that there is some physical activity that threatens your well being such as display of a weapon, multiple assailants, or abnormal aggressive behavior. 
