I live in Oklahoma City and we would have armed vigilante groups roaming the city on the look out for more muzzies.
I live in NW Iowa just sixty miles from Sioux Falls, SD., a sprawling, big town/ little city with a greater population of 150k or so and its a business/recreational/health care magnet for SW Minnesota, NW Iowa, and NE Nebraska. They also have many large recreational venues for sports and other entertainment.
This summer my wife and I attended an Eagles concert with a packed house of 18,000. As I wrote the Govenor of SD yesterday, these are exceedingly soft targets for a group that wants to hit the heartland. Of course ticket holders' small bags were checked on entrance but no CCW check. But it would be easy to bring bombs or AKs in in some manner before hand by a determined person(s) and with a crush of that many people with few aisles, an AK spraying from the balcony could kill and injure hundreds in minutes. Best scene scenario, you are there with your CC weapon in a crush of panicked folks maybe ten yards from a shooter, maybe one fifty; they with large capacity magazines, you with maybe a ten round mag. It would like bringing a knife to a gun fight and it could be like Paris or worse all over again.
As of yesterday when I emailed the SD Govenor (Sioux Falls was one of cities slated to receive "Syrian" refugees) they were not on the list of states to refuse refugees. I emailed the Iowa Govenor also and they are on the growing list of states refusing refugees presently.
But back to the original question, in a large, packed venue described, much would depend on how many folks were carrying, their proximity to the shooter(s), their frame of mind, and capability. It could be very ugly very fast just like in Paris and I cannot see these Islamists, if they were planning a truly big event, not picking a target where there is maximum exposure to the most people in the shortest time possible. And with automatic weapons and explosives.
