While nothing is "invulnerable"; Modern body armor is extremely effective.
As suggested by Saletan, here is the U.S. Justice Department’s breakdown on body armor...
Armor Classifications for Ballistic-Resistant Armor
NIJ Standard–0101.04 establishes six formal armor classification types, as well as a seventh special type, as follows:
Type I (.22 LR; .380 ACP). This armor protects against .22 long rifle lead round nose (LR LRN) bullets, with nominal masses of 2.6 g
(40 gr), impacting at a minimum velocity of 320 m/s (1050 ft/s) or
less, and against .380 ACP full metal jacketed round nose (FMJ RN),
with nominal masses of 6.2 g (95 gr), impacting at a minimum velocity
of 312 m/s (1025 ft/s) or less.
Type I body armor is light. This is the minimum level of protection
every officer should have, and the armor should be routinely worn at
all times while on duty. Type I body armor was the armor issued
during the NIJ demonstration project in the mid-1970s. Most agencies
today, however, because of increasing threats, opt for a higher level
Type II-A (9mm; .40 S&W). This armor protects against 9mm full metal jacketed round nose (FMJ RN) bullets, with nominal masses of
8.0 g (124 gr), impacting at a minimum velocity of 332 m/s (1090 ft/s) or less, and .40 S&W caliber full metal jacketed (FMJ) bullets,
with nominal masses of 11.7 g (180 gr), impacting at a minimum
velocity of 312 m/s (1025 ft/s) or less. It also provides protection
against Type I threats.
Type II-A body armor is well suited for full-time use by police
departments, particularly those seeking protection for their officers
from lower velocity 9mm and 40 S&W ammunition.
Type II (9mm; .357 Magnum). This armor protects against 9mm full metal jacketed round nose (FMJ RN) bullets, with nominal masses of
8.0 g (124 gr), impacting at a minimum velocity of 358 m/s (1175 ft/s) or less, and .357 Magnum jacketed soft point (JSP) bullets,
with nominal masses of 10.2 g (158 gr), impacting at a minimum
velocity of 427 m/s (1400 ft/s) or less. It also provides protection
against Type I and Type IIA threats.
Type II body armor is heavier and more bulky than either Types I or
II-A. It is worn full time by officers seeking protection against
higher velocity .357 Magnum and 9mm ammunition.
Type III-A (High Velocity 9mm; .44 Magnum). This armor protects against 9mm full metal jacketed round nose (FJM RN) bullets, with
nominal masses of 8.0 g (124 gr), impacting at a minimum velocity of
427 m/s (1400 ft/s) or less, and .44 Magnum jacketed hollow point
(JHP) bullets, with nominal masses of 15.6 g (240 gr), impacting at a
minimum velocity of 427 m/s (1400 ft/s) or less. It also provides
protection against most handgun threats, as well as the Type I, II-A,
and II threats.
Type III-A body armor provides the highest level of protection
currently available from concealable body armor and is generally
suitable for routine wear in many situations. However, departments
located in hot, humid climates may need to evaluate the use of Type
III-A armor carefully.
Type III (Rifles). This armor protects against 7.62mm full metal jacketed (FMJ) bullets (U.S. military designation M80), with nominal
masses of 9.6 g (148 gr), impacting at a minimum velocity of 838 m/s
(2750 ft/s) or less. It also provides protection against Type I
through III-A threats.
Type III body armor is clearly intended only for tactical situations
when the threat warrants such protection, such as barricade
confrontations involving sporting rifles.
Type IV (Armor Piercing Rifle). This armor protects against .30 caliber armor piercing (AP) bullets (U.S. military designation M2
AP), with nominal masses of 10.8 g (166 gr), impacting at a minimum
velocity of 869 m/s (2850 ft/s) or less. It also provides at least
single-hit protection against the Type I through III threats.
Type IV body armor provides the highest level of protection currently
available. Because this armor is intended to resist “armor piercing”
bullets, it often uses ceramic materials. Such materials are brittle
in nature and may provide only single-shot protection, since the
ceramic tends to break up when struck. As with Type III armor, Type
IV armor is clearly intended only for tactical situations when the
threat warrants such protection.
- Types I, II–A, II, and III–A armor are required to prevent
penetration from the impact of six bullets per panel, for two
complete samples (front and back panels) at specified velocities and
locations for two types of ammunition. Two of the impacts in each
six-shot sequence must be at a 30-degree angle. A total of 48 shots
are completed on four samples. Furthermore, the deformation of the
backing material (a measure of blunt trauma protection) must not
exceed 44mm (1.73 in). Deformation readings are taken on each panel
at shot location 1, then at either shot location 2 or 3, whichever
one had the highest shot velocity. The armor must meet these
requirements while wet.
- Type III armor requirements are identical to those above, except that
only one type of ammunition is specified, and all six test rounds are
fired perpendicular to the surface of the armor. A total of 12 shots
are completed (6 shots per sample).
- Type IV armor is required to resist penetration from only a single
type of ammunition (armor piercing) and is only required to prevent
penetration and backface deformation greater than 44mm (1.73 in) from
a single perpendicular impact. A total of two samples are tested.
- The maximum allowable deformation of the clay-backing material was
determined through an extensive series of ballistic gelatin
measurements and experiments conducted by a team of medical experts.
This limit ensures protection from blunt trauma that arises from an
impact occurring over vital locations. Even this level of protection,
however, does not give an absolute guarantee of protection against
Additional protection from impact blunt trama:
- Trauma plates are devices that can be added to the vest over a localized area (most commonly the mass center of the torso) to
increase the wearer’s protection against blunt trauma injuries. Blunt
trauma injuries are caused by the impact forces of the bullet against
the armor, resulting in nonpenetrating internal injuries such as
bruises, broken ribs, or other injuries to internal organs. Trauma
plates can be made of a hard substance such as metal wrapped in
rubber or ballistic fabric, or they can be made of additional layers
of ballistic fabric, similar to an armor panel. Some manufacturers
even build trauma plates into the armor panel itself.
According to the UCR data from the period 1990 to 1999, 290 law enforcement officers were killed while wearing protective armor (see exhibit 11). Of those officers 160 (55.2 percent) were killed by gunshot wounds to the head; 101 (34.8 percent) died as a result of gunshot wounds to the upper torso; 18 (6.2 percent) died as a result of gunshot wounds below the waist; 5 (1.7 percent) were struck by automobiles; 2 (0.7 percent) were stabbed; and 4 (1.4 percent) died by other means.
Of the 101 officers killed by gunshot wounds to the upper torso, 40 (39.6 percent) were killed when the round entered the torso region between the panels of the vest or through the arm openings, and 34 (33.7 percent) were killed when the round landed above the coverage area of the vest.
Twenty of the 101 officers killed by gunshot wounds to the upper torso died as a result of rounds penetrating the body armor. Of these 20 incidents, all were the reported result of rifle rounds, which the armor was not designed to protect against. It is important to note that no documented fatal injury has ever resulted from a round of ammunition penetrating body armor that NIJ had approved as protection against that level of threat.
Source: Selection and Application Guide to Personal Body Armor, U.S. Department of Justice (2001).
"Shock Trauma" from impact forces
There is evidence that even without any armor humans can be pretty hard to stop:
Short of disrupting the brain or severing the upper spinal column,
immediate incapacitation does not occur. Therefore, the threat remains
to the officer. Yet, implicit in the media presentations of law
enforcement encounters is the belief that with the “proper handgun”
and the “proper ammunition,” officers will inflict immediate
incapacitation if they shoot offenders anywhere in the torso. Varied
and multiple real-life law enforcement experiences contradict this
false and dangerous belief.
In the authors’ ongoing study of violence against law enforcement
officers, they have examined several cases where officers used
large-caliber handguns with limited effect displayed by the offenders.
In one case, the subject attacked the officer with a knife. The
officer shot the individual four times in the chest; then, his weapon
malfunctioned. The offender continued to walk toward the officer.
After the officer cleared his weapon, he fired again and struck the
subject in the chest. Only then did the offender drop the knife. This
individual was hit five times with 230-grain, .45-caliber hollow-point
ammunition and never fell to the ground. The offender later stated,
“The wounds felt like bee stings.”
Source: One-Shot Drops: Surviving the Myth, FBI Law Enforcement
And when wearing armor...
So imagine your disbelief when a threat continues to progress-even
after you've delivered what would normally be considered
"incapacitating" rounds. You think to yourself, Is it the gun? The
ammo? Did I hit or miss? I had to have hit, but he keeps coming.
Imagine the thud of realization when you determine your adversary is
wearing body armor.
Source: Bullet Proof: What do you do when the bad guy seems invincible?
And definitely no sign of "shock trauma" in the 1997 North Hollywood shootout:
The patrol officers were armed with standard Beretta 92F and Beretta
92FS 9mm pistols and Smith & Wesson Model 15 .38 caliber revolvers,
Officer James Zaboravan also carried a 12-gauge Ithaca Model 37
pump-action shotgun, but the body armor worn by Phillips and
Matasareanu was strong enough to resist penetration.
In this case, approximately 650 rounds were fired at two very heavily
armored men, who had fired approximately 1,100 rounds. The responding
police officers directed their fire at the "center of mass," or
torsos, of Matasareanu and Phillips. Each man was shot and penetrated
by at least ten bullets, yet both continued to attack officers.
Source: North Hollywood shootout, Wikipedia
Multiple hits only seemed to give these two guys a direction to shoot in.
I could find no other evidence that the force of bullet impact(s) would "have disabled the man long enough for him to be rushed and overpowered".
All things considered:
- An "armed populace" is not going to have much effect on a determined
psychopath in modern body armor.
Ps. I'm not taking a stance on guns here so please don't start that debate.