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We see it in almost any article, write up or review of body armor “multi-hit rated”. It’s also written into the NIJ standards for body armor, but what exactly does it mean and why does it matter to you?
Multi-hit rating
Essentially a multi-hit rating is pretty self-explanatory. It means that a particular piece of body armor is designed to protect the user and successfully prevent multiple projectiles from entering the body. But, unfortunately it isn’t quite that simple. There are a ton of factors that dictate whether a projectile has the ability to penetrate body armor and a statement like “multi-hit rating” doesn’t even begin to cover the black hole of information that need to be discussed to dissect the validity of that statement.
For example some basic ¼ inch sheet steel would prevent multiple hits from a 9mm, .22 or even .45 from penetrating the plate, but it would fail miserably with just one round of .556 green tip, or a 6mm rifle cartridge with a velocity higher than 2900 fps. So whenever you see a statement that vague, make sure you ask the question “against what” and “how many is multi”. Both of these questions will help you to determine if that armor will provide you the protection that you are expecting it to.
Why is a multi-hit rating important?
Now that we have covered what multi-hit means (or doesn’t mean) now let’s deep dive into why that rating is important to you the consumer.
Training
When you look at the firearms training that is administered to any member of the professional firearms community you will hear terms like controlled pair, hammer pair, double tap, shoot until the threat is down, or failure to stop drill. The one thing that all these techniques have in common, is they all require the shooter to fire at the threat more than one time. This is required because it increases the likelihood of an effective hydraulic wound, mechanical failure of the body and shock to the system of the aggressor. These results of terminal ballistics are what is going to render the threat inert and are unlikely to be accomplished with just one shot. A hydraulic wound is any wound that causes a severe “leak” in the circulatory system. This type of wound can absolutely be fatal, but in the timeline of a shooting, it is very slow and does not immediately neutralize the threat.
Think of it like a fluid leak in your car, it can be a simple as an old gasket or cracked hose, or as bad as a cracked oil pan that will drain out in seconds. While catastrophic, and will result in the car failing, it is not instantaneous and the car will still make it hundreds of yards if not miles before grinding to a halt.
The human body is no different, depending on the severity, it can take anywhere from 45 seconds to hours for someone to bleed out.
Now, mechanical failure is when something vital to your skeletal structure is compromised or destroyed. Again, this could be fatal over time, but is more likely to incapacitate the threat since it can prevent their body from functioning properly.
Sticking with our car comparison, this would be like your axel snapping in half. It would quickly stop the car in its tracks, but the engine is still running and functioning.
Which would bring your car to a stop faster, a fluid leak or a broken axel?
The final effect that can be caused by multiple shots is hydrostatic shock. This is the shock to the nervous system due to pressure changes in the body from a projectile penetrating tissue at high speeds. This damage can be so extensive, that studies on animals that were wounded in the thigh, showed damage to the brain.
One shot, may cause long term neurological damage but will not immediately send the system into shock. Multiple rapid shots on the other hand has the ability to send the system into a spiral and cause temporary incapacitation of the individual being shot. Imagine one sensor on your car going out, depending on the sensor your car might stall or go into limp mode, but it will probably just put on the check engine light and keep on going. Now if every sensor in your car went out in rapid succession, your car would immediately shut off even if it was mechanically still running.
When you look at the firearms training that is administered to any member of the professional firearms community you will hear terms like controlled pair, hammer pair, double tap, shoot until the threat is down, or failure to stop drill.
The one thing that all these techniques have in common, is they all require the shooter to fire at the threat more than one time. This is required because it increases the likelihood of an effective hydraulic wound, mechanical failure of the body and shock to the system of the aggressor. These results of terminal ballistics are what is going to render the threat inert and are unlikely to be accomplished with just one shot.
A hydraulic wound is any wound that causes a severe “leak” in the circulatory system.
This type of wound can absolutely be fatal, but in the timeline of a shooting, it is very slow and does not immediately neutralize the threat. Think of it like a fluid leak in your car, it can be a simple as an old gasket or cracked hose, or as bad as a cracked oil pan that will drain out in seconds. While catastrophic, and will result in the car failing, it is not instantaneous and the car will still make it hundreds of yards if not miles before grinding to a halt.
The human body is no different, depending on the severity, it can take anywhere from 45 seconds to hours for someone to bleed out.
Now, mechanical failure is when something vital to your skeletal structure is compromised or destroyed. Again, this could be fatal over time, but is more likely to incapacitate the threat since it can prevent their body from functioning properly. Sticking with our car comparison, this would be like your axel snapping in half. It would quickly stop the car in its tracks, but the engine is still running and functioning.
Which would bring your car to a stop faster, a fluid leak or a broken axel?
The final effect that can be caused by multiple shots is hydrostatic shock. This is the shock to the nervous system due to pressure changes in the body from a projectile penetrating tissue at high speeds. This damage can be so extensive, that studies on animals that were wounded in the thigh, showed damage to the brain. One shot, may cause long term neurological damage but will not immediately send the system into shock. Multiple rapid shots on the other hand has the ability to send the system into a spiral and cause temporary incapacitation of the individual being shot. Imagine one sensor on your car going out, depending on the sensor your car might stall or go into limp mode, but it will probably just put on the check engine light and keep on going. Now if every sensor in your car went out in rapid succession, your car would immediately shut off even if it was mechanically still running.
Application of knowledge
So, now that we have gone over the training that is applied by individuals who are comfortable with firearms, how do we apply that to our armor set up?
First we have to identify what level of armor we need for our individual set up. This is dictated by a number of factors, but the main one is what does it need to stop? Just saying “I want it to stop everything” isn’t always practical, if you are running a low profile protective detail, is it necessary to have level 4 plates in a vest with soft armor? No, probably not, since this would defeat the purpose of being low profile.
Additionally, with steel plates particularly, weight can also become an issue. Larger, higher rated plates typically add weight to the setup, which may not always be practical depending on the situation. It is recommended to tailor the setup to its particular task. So, first you want to identify what type of projectile you need it to stop, and then determine if it is rated for multiple impacts from that projectile.
What is the best way to determine this information you may ask? Well, the National institute of Justice (NIJ) ratings for ballistic resistance of armor. Reading over the document you will find a breakdown of what each level of armor protects against and how many shots from each of those calibers it will defeat to meet that rating.
4.1.2 Hard Armors and Plate Inserts
All hard armors and plate inserts shall be subjected to a 24 shot P-BFS test and to either a 24 shot or a 12 shot BL test. All hard armors and plate inserts shall be conditioned per section 6 prior to ballistic testing. All hard armors and plate inserts shall be no larger than 254 mm x 305 mm (10.0 in x 12.0 in) for testing. The required number of armor samples is dependent on the armor type, as described in the following sections.
4.1.2.1 Type III
For hard armors and insert plates intended to provide Type III protection, the compliance test group shall consist of nine armor panels. The armor panels shall be sufficiently large to allow for a minimum of six shots per panel. These requirements are outlined in figure 5. Four armor panels will be used for the P-BFS testing described in section 7. A minimum of four armor panels will be subjected to the BL test described in section 7, with a minimum of 24 shots. The remaining armor panel is a spare and be used if necessary”.
Things to consider
Now that we have gone over what a multi-hit rating is and why it’s important, let’s discuss some of the grey area that may not be defined yet.
If my armor is multi-hit rated, does that apply to all calibers? The short answer is a resounding, no. As you can see from the test criteria, they do not use a wide variety of calibers when testing armor. This would be a very long and extremely expensive process, if each plate had to be shot multiple times from every known caliber. Instead, they chose to focus on the most commonly available calibers that may be encountered by the end user.
You may find some YouTube videos testing armor with different calibers or heavier/lighter projectiles, but take those with a grain of salt. These tests are not scientific and rarely are conducted in a realistic manner. Most of the time the tester only has one or two sets of those plates and needs to show something catchy to draw in viewers. Just because it stops one of “insert random caliber”, does not mean that the plate is now rated for that caliber. Make sure to thoroughly research and validate, each product and claim before trusting your life to some “internet testing”. These videos do have some useful information, but it is best extrapolated with an understanding of the actual testing criteria, and what is realistic, then just “it looks cool”.
We have covered the importance of having armor that is rated for multiple impacts, the process behind how it is tested, and how that testing may fall short in real life. We also briefly covered how to not be drawn in by catchy YouTube gimmicks on why armor may or may not be effective against certain ammo types. But, if in doubt, remember the five D’s of a gunfight “dodge, duck, dip, dive and dodge”, and if all else fails, make sure your plate will survive long enough to get you off the X.