I was a sort- of witness to one, but didn't realize guns were involved until later.
Explanation:
Almost 20 years ago, when my younger brother was in middle school and I was a freshman in high school, a gang of young thugs came to our house while our parents were out at a movie, angrily claiming that my brother had stolen money from one of them. For the first part, they were angrily talking to my bro while I watched from a distance, far enough away that I wasn't intruding on their conversation (It wasn't my business, and at the time I didn't realize how serious it was) yet near enough that I'd be able to intervene if things got ugly. Eventually, they made my brother let them come inside the house (Keep in mind he was maybe 13 years old at the time, if that, and we were outnumbered at least 2 to 1), hung around for a little while, then left. When they were gone, we of course called the cops, and my bro revealed that they made it clear to him that at least one of them was packing, and threatened to "Spark" him. Plus, one of the thugs stole my Game Boy while they were inside. The thugs were eventually caught and jailed (My parents were at the trial, and I remember my dad saying he saw them come into the courtroom wearing handcuffs and leg- irons like in 'The Green Mile'), and I got my Game Boy back after a while, but looking back, it scares me how close we could have come to losing our lives.