Everyone has their opinion on how to respond, if you should respond, and the best equipment to respond to an active shooter situation. I’ll give my opinion on the equipment I've selected and my thoughts on responding.
Additional side note.
Consider your everyday attire, dress in a way that is common for your area/region you will be in. Consider wearing footwear that isn’t screaming tactical but also provides some protection and maneuverability. I’m not a fan of wearing flip-flops in public because they are only going to cause issues for you.
Consider that any situation where law enforcement shows up, everything you are wearing/using will be under scrutiny. If you have Punisher logos all over your gear and clothing, expect that to be something focused on as you may be seen more as a vigilante vs. a good guy with a gun acting to protect yourself or others.
Moving on to the equipment I've selected and some the rationale behind the set ups.
I carry a basic gun belt/war belt/battle belt. I have a holster that will accept my EDC (G17), two pistol magazines, two AR magazines, lock blade knife, multi-tool, IFAC containing compression gauze, hemostatic gauze, tourniquet (TQ) (total of five TQs carried between plate carrier (PC) and gun belt).
I also have a mix of keepers on the belt, should I have the time I can apply the keepers to secure the gun belt to my under belt. The gun belt is tight enough to not move around during vigorous activity but keepers are highly preferred. Make sure to train with any equipment you set up. You’ll notice what seems to be a good idea in theory is often needing some tweaking in practice. Make sure you don’t have any gear placed in such a location you can’t effectively take a knee, get into the prone position. Stress test your equipment in training. You’ll be surprised at the yard sale that happens under heavy use. Make changes to secure and silence your equipment accordingly.
I've opted to go with a low profile PC. I’ve outfitted the PC with the lightest plates I can afford, ShotStop GT2 level III special threat. Rated to stop multi-hit from SS109 and M855. These plates weigh in at only 3.8 pounds each. The PC is light, low profile and I'm highly maneuverable in it. I have no problem getting very low into the prone position with this PC. I only carry three AR mags on the front of the PC allowing the other pouches to carry additional compression bandages and four tourniquets, these can be used for immediate self-aid as throw devices should the threat still be present and you are pushing forward through casualties. The abdominal pouch (Dangler/Hanger/Gut Hider) carries additional compression bandages, hemostatic gauze and the fourth tourniquet. In total, the fully loaded PC weighs a bit over 13 pounds.
Another addition I've added to the plate carrier is a front and back ID placard. I’ve seen several variations such as Security, Active Response, Agent, you need to conduct your own research to determine if such ID placards may be considered impersonating law enforcement in your area. The biggest thing we are trying to do with the placard is give a moment of pause to any first responders, providing a visual clue we are not the active killer, but rather a good guy with a gun.
I have a basic AR pistol set up with an 11.5 barrel, 1x8 low power variable optic (LPVO), offset Halosun HE407-GR-X2 reflex sight, SureFire M600 Scout light, Steiner DBAL-A3. I carry the AR pistol in a “patrol ready” configuration. Between the gun belt, PC, and mag in the weapon, I will turn out with a total of six magazines. This is more than enough in my opinion for an active shooter event in which a citizen responds. We are only trying to address the time between the incident start and follow on resources, such as local Law Enforcement. One of the biggest considerations for our equipment is to have a balanced kit that takes into account the total weight and bulk of equipment. Lightweight kit is highly recommended, it allows you to be more maneuverable and you won't gas as quickly. This is another opportunity I'll push that you should be training with your equipment. Your endurance is just as important as quality equipment.
I found a perfect use for some of the AR500 armor plates I’ve acquired over the years, I've added four plates into an extra-large PC. This helps me work those muscle areas specifically used when wearing kit and holding weapons at shoulder level. You'd be amazed at how weak most people are when wearing a PC and attempting to mount a rifle to their shoulder for any amount of time.
I also have an eight magazine bandolier available, this is not part of my considerations for response gear, this is more an option available for a “get home” situation where a total of six AR magazines may not be providing the capacity we would like to have. I’ll be posting more on get home bags and Grey Man considerations soon.
Equipment is a very small part of the overall considerations of responding to an active shooting event. Plan accordingly, mental model the before, during, and aftermath.