If it is a full on terrorist attack with multiple shooters using rifles you don't necessarily have to be out-gunned. You only need to get the first one. You can then take his weapon. There are risks with "upgrading" in this way though too. One, you just took him out, so obviously this means you have to go to an exposed position to retrieve his weapon and ammo. Two, when law enforcement arrives and/or other CC you are going to look like one of the terrorists since you are now similarly armed... So you might want to simply maintain cover and selectively engage...
The real message here is, you need to think about these things ahead of time. Once a situation unfolds, you're not going to have the time nor concentration to weigh these options. If you're carrying, you need to have at least an outline of when you feel you could/should engage, and when you would simply stay out of it.
If you can afford it, take a practical shooting course. Then join a practical shooting club.
Well said.
Pray,at all times. Carry, at all times. Never give up, at all times. Practice, practice, practice...