As Gary Jules put it, "it's a very very mad world". Nutcases get guns illegally and then go shoot up malls, synagogues, churches, and schools. The worst part is that the law abiding are not always allowed (despite the 2nd Amendment...) to carry guns to defend themselves. Even if you are lucky enough to be carrying a gun when the poop hits the oscillating metal blades, you need to know what to do to stay alive. And punching holes in the 10-ring of your 25 yard NRA pistol target, while a good start, is not everything. Because engaging an active shooter is an even more nebulous subject, this article will focus on evasion.
You're walking through your local mall when all of a sudden, gunfire erupts. Now your first instinct might be to locate the fire and run away. You might get away with that if the crowd is big enough and the fire is far enough away.
What you should do is hit the floor immediately. Then, if you can, get a general direction of the fire and find something concrete or thick metal to get behind. Most malls here in the midwest are made of brick or some kind of compressed concrete. About 6" worth of that should stop most handgun rounds and most shotgun rounds. A rifle, however, may penetrate it and a burst or two certainly will. See the fountain above? If you put that between you and the threat and crawled away as quickly as possible, that thing would take a lot of fire for you.
Dry wall will not stop much of anything. Most common handgun calibers can penetrate six to ten thicknesses. Shotguns and light rifles can penetrate much more. Drywall and wall boards are not cover, but they can conceal your escape. Stay low, and keep out of the threat's line of sight. Move fast, and if you get shot, don't stop and whine about it. If you can still move, then move. Modern medicine can put you back together from pretty serious damage. Flesh wounds to your extremities are nothing to worry about immediately. Keep going and feel the pain later. You can only get medical help if you're out of the shooter's range.
Cars are not cover either. The engine compartment will provide some cover, as will the wheel rims, but if the shooter has a rifle, they may not be enough. Car bodies are easily sliced up by even light pistol cartridges. If you get stuck behind a car, put the engine compartment between you and the threat, and protect the rest of your body by staying in close alignment to a wheel rim if you can. Stay low and crawl on your belly to other positions, going under other cars if you can instead of around or over them.
Tables, chairs, and other furnishings are not cover either. All of them will be easily shredded by anything that goes "boom". These things can conceal you, but if you draw fire while behind them, you will be hit, and the bullet won't even slow down much. You have to be smart about using concealment. There are too many variables for me to give any good universal advice. Just keep your cool and pick a fast, protected route the hell out of there.
If you must run from one point to another, do so in short bursts. The Army preaches 3 to 5 seconds, with your sprint ending at a new covered or concealed position. If you get a chance to run longer, i.e. the threat starts moving in the other direction or stops paying attention to you, then run, run, and run some more--keeping near covered or concealed positions. Have an idea of what you're going to dive behind if the threat turns his attention back to you. Being fat is not an advantage here. You don't have to look like Arnold, but being able to sprint a few hundred yards without pooping your pants will be very helpful.
Another idea of note is that once the shooting starts, all rules go out the window. If you need to kick down a door or smash a window to get out, then do that. Don't break glass with your body. Use a chair, a baseball bat, a large potted plant, even a very large book might do. Common fire extinguishers are great for breaking glass. If you must break glass with your hands or feet, wrap the appendage doing the breaking in heavy cloth or leather. You're still probably going to get cut. Oh, and kick doors near the door knob, as most locking mechanisms are found there. Being fat will help you break the door down, as more weight is usually better. Kick like you mean it. Steel doors or doors with reinforced jambs will probably not break, so you're better off finding somewhere else to try. At this point your goal is to survive, not to be polite. Break what you have to if it gets you out alive.
I hope none of you ever need this advice. Stay tuned, as I may do a short write up on engaging an active shooter, but there are a HUGE number of variables there and it may take my tiny caveman brain some time to pare it down to something useful and intelligent sounding.