I'm an individual who is pro gun, but I agree with your assessment. A very well trained individual with a very high level of situational awareness would still have had to be in a location to respond effectively, and I'm curious, would a bullet strike actually detonate the vest? That said, if one attacker of three was taken out, there might still be less death. There are many situations in the US where armed individuals have stopped potential mass murders underway, though they weren't going up against trained terrorists (at least in the stories I reviewed), and in a few of the examples, the individuals that stopped the murders underway were actually off duty, armed LE...well trained with their weapons.