I'm going to put in a vote for NOT going with a bagged AR pistol. Instead, I'm going to suggest a semi-auto handgun with a red-dot sight on it. The main reason is speed of action. The bag gun has to be removed from the bag and the LAW unfolded before it can be brought to bear. The handgun has to be drawn and then it's up and on. The difference in that time in your case is measured in killed and wounded. Also, if you have to maneuver, especially through a crowd, with your gun in hand it will be easier to protect a handgun and move safely with it than with an AR pistol. It will also be easier to clear and bring to bear the pistol when you're coming from a sitting position in a pew or if others are in close proximity when the attack starts.
As of dealing with armor, the AR's effectiveness depends on whether the attacker has soft armor or is wearing rifle plates. If it's rifle plates you have to do the same thing with the AR pistol as you do with the regular handgun--shoot where the armor is not. The red dot on the handgun will make this much easier, make precision shots easier and faster, and allow you to engage at distance with more confidence. A rifle or AR pistol somewhere as a support weapon is not a bad idea, but as the initial response tool I would go with the handgun. Caveat is that some of these shooters were seen when they were still outside approaching the church. In that case, the AR pistol is the better choice.
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