I keep a trunk gun. It's legal to keep it there so long as it's stored correctly. Assuming an active shooter scenario or loved ones threatened and tehy are are unable to escape and i used it, am i likely to get pasted by the law anyway? Most situations if i can get to my car, i can leave the scene, and that's the choice I'd take. My oifficial explanation is that there's lots of rabid animals in my AO and I keep it just in case. (Which is true actually)
First the trunk gun.
Yes it is legal, but as long as you are following storage regs not transport regs. There have been precedent setting cases where people have been charged and convicted for improper storage for assuming that just because its in your car its in transport. If you are not currently on your way to or from the place you plan to use the firearm, it is in storage.
Second the active shooter.
The answer is depends. I don't think you would be charged in any variation for ending the threat. But that is not your only issue in this scenario.
Bottom line is if you arm yourself in an active shooter situation and attempt to move to the shooter/shooters you are most like going to be shot and killed by the police. It is not your job, responsibility or even wise to attempt this. All this will do is tie up police resources dealing with you, or most likely your corpse. Which will ultimately delay the response that your loved ones may need in a timely fashion.
There are various things that are in place to help the police not shoot each other and undercover officers in these situations. Such as comms with dispatch, the color of the day and a few others.
Current training is that the first 2 officers on the scene are coming in and they are going to be moving to the gunfire. If you are seen with your truck gun you most likely will be engaged and shot until you are on the ground, at which point you will be handcuffed and position reported and possibly left there until the real shooters are dealt with. You will not be given medical aid until the area has been deemed clear and safe for the paramedics to enter.
Now to add to the above these things happen in public places such as malls and schools. Think about how long it takes you to walk around your local mall or school and times that by 10. That is how long you will be waiting for medical aid as the mall/school is searched and cleared if that phase happens right after you are engaged. Which is unlikely as you would not be the bad guy.
Now if this happens in the parking lot and you are at your car, engage and end the threat. But be prepared to be arrested and held while everything is figured out. And for the love of god open the action of your gun and lay it on the ground as soon as you hear sirens close to you and raise your hands or we may be back to the above. Do exactly as you are told when you are told without question. Once you are cuffed and the adrenalin is running down is the time to talk to the police and explain your position.
You best bet is that if you need to get to your car to get the gun, you should be leaving the area, as you just dragged your loved ones to your car. If you are not in the same location you best bet is to let the police handle it, as you will most likely get you loved ones killed by the bad guy as the police are dealing with you, because you now look like a threat.
Hope that gives you a little to think about