Some great info here from a former Navy Seal. I wrote up Cliff Notes for those who prefer to read.

How to Survive an Active Shooter

Three things you must do if an active shooter is in your vicinity:

1. Run
2. Hide
3. Fight

Your environment, the situation, and your personal capabilities will decide which one you do.

RUN

Figure out where the shots are coming from — In enclosed spaces, sounds get distorted: a sound coming from the left can sound like it’s coming from the right. And just because you see a crowd of people running by doesn’t mean they’re running in the right direction. Trust your eyes — and question your ears.

Know your escape routes — Think them through ahead of time. Whether you’re at work, home, coffee shop, gym, or any place your frequent, ALWAYS think about how you would escape.

Run in a zigzag pattern — This is critical for wide, open areas (like parking lots) or if you’re the only one running down a hallway and you know the shooter is right behind you. Be sporadic, go left and right, go high and low. Make it hard for the shooter to lock you in.

Don’t run with everyone else — Take a moment to look at, listen to, and smell your environment. Know where the shooter is and ensure you’re going to run in the right direction.

Run from cover to cover — Think of running in terms of short sprints, not a marathon. Choose:
• Concealment – Takes you out of sight but doesn’t stop bullets (e.g., hiding behind a curtain).
• Cover – Takes you out of sight AND stops bullets.

HIDE

Find the hiding spots that also stop bullets — Such as a granite table or behind the engine end of a vehicle.

Find hiding locations in advance — Determine what gets you out of sight but is also not a dead end.

Avoid dead ends in hiding places — Bathrooms are a dead end; there’s no way to properly barricade the door and there’s no other way to get out.

Be able to barricade the door and escape — If can be a hallway that has multiple points of egress and entry; just be sure your hiding spots give you other opportunities of escape.

Barricade the door first — Stack barriers linearly: set a table up against the door, then another piece of furniture behind the table, and so on, all the way to the opposing wall, which will then act as a doorstop.

Keep your body away from the center of the door — Bad guys like to shoot through doors, so when you barricade a door, do it quickly and without putting yourself dead center in front of the door. Once the barricade is set, immediately look for an exit.

Make the room dark and remain silent — Put your cell phone on silent (better, on Do Not Disturb to also turn off vibrations). Turn off all the lights. Lock other doors, windows, or any other way in — and make sure those are barricaded, as well. Get quiet.

Keep your body away from doors and walls — Assume the shooter will shoot through doors and walls. Get as low or as high in the room as possible (lie flat or climb on a fridge). Keep your head and torso out of the line of fire.

FIGHT

Team up with others — Find others who are willing to fight the shooter. Make a quick plan, such as: You tackle his legs/hips and I’ll grab the gun.

Gain control of the gun first — Grab it like a pull-up bar, a chin-up bar, or an over-under. Whatever you do, get a positive grip on that weapon and then let gravity do the rest: drop to the ground. The other person is going to take out the body.

Gain control of the shooter’s spine — Take out the shooter’s hips.

Fight for your life — Use big movements such as tackles and grabs. Use the gun or any improvised weapon at your disposal against the bad guy: fire extinguisher (spray him in the face or hit him over the head), cleaning products (spray in eyes), scissors (stab him in the face). Whatever you do, go 100% — because it’s either you or him.

The fight is all about creating pain, disarmament, and then taking down the shooter. Once you do so, maintain distance, restrain the shooter if necessary, and wait for first responders. Make sure the shooter is neutralized before helping victims.