Might have been a nice hunting rifle in the trunk of a passing car or nearby parked car. What do you think was going through the mind of at least half of those country music lovers. Get to my car. Get my gun.
Some people might have thought about it but it would not have been a good idea. If people at the concert started shooting at a distant hotel and those bullets reached the hotel they could have passed through hotel windows and killed or injured more people. The police might have taken out anyone that did that. Not too good of an idea.
I imagine anyone with firearm during a mass shooting event would've been immediately shot by the police. Or shot by another country music lover who went to his car to get his gun.