A bullet has high lethality (is highly lethal) if its projectile does great damage to the flesh of a living body. Extremely high projectile speed can result in high lethality. Without such speed, large projectile diameter and weight can result in high lethality. Given a projectile speed, diameter and weight, the lethality can be increased by doing things which cause the bullet cross-section to increase in area as it goes through the body. For example, this is done by making the projectile with a soft nose and with devices such as hollow points or wedges.
Why would anyone want to make a bullet more lethal? First, for hunting bullets, it is desirable for bullets to be highly lethal so that an animal that is shot dies quickly so that it will be taken home for food rather than "getting away" to suffer in various ways before finally dying.
When it comes to defensive use, the reason is the same for police as for any person needing to defend the life of self or someone else. When facing an armed violent criminal, police don't shoot at the criminal so as to "wing" or cripple him or shoot the gun out of his hand like in the old westerns. They don't because extremely few people can shoot that well while moving and/or while the criminal is moving. Taking such a shot makes a miss too likely. A miss means the criminal gets additional time to hurt, and maybe KILL, someone (like you).
To protect themselves or others, police shoot to kill when they have to shoot. They are taught, just as anyone else is taught in regards to using a gun for self defense, that a person must not have a gun for self defense unless the person is prepared to shoot to kill if the person ever has to use the gun in defense. A person not so prepared endangers his own and other lives more than if he has no weapon for defense.
Police and others using a gun against an armed criminal must take the highest probability shots so as to yield the highest probability that the criminal will not shoot (or stab, or club) anyone else. The highest probability that the criminal won't hurt anyone else is achieved by taking shots most likely to kill the criminal. The police aim for the center of the chest. They use bullets that they hope will do serious damage, although some forces limit them somewhat in this regard.
If the bullet projectile speed is extremely high and the projectile is relatively pointed and hard, the projectile will tend to pass through a material like flesh and still retain a high speed and energy. The shock wave in front of the projectile and the suction (vacuum) caused behind it by its rapid passing can cause extensive damage to a material like flesh.
If the speed is not so great, a pointed, hard (copper jacketed) projectile tends to pass through a material like flesh without causing much damage. This is typical of most of the assault weapon calibers, and of the calibers of most of the semiautomatic rifles that gun control advocates and existing laws call "assault weapons."
These are the rifles of .222 and .223 calibers. The bullets have muzzle (coming out of the barrel) energies of about 1140 and 1330 foot-pounds when fired out of single-shot test barrels, so they have somewhat less when fired from a semiautomatic.
This means that most "assault rifle" bullets do relatively little damage to a person, assuming the projectile goes through flesh or nonvital organs. This is in comparison to calibers that armies used in the past, and in comparison to calibers used for big-game hunting. And these are the same bullets as used extensively now by our military and others.
The fact is that armies have learned that they don't need or even want weapons to kill enemy soldiers. On the battlefield putting enemy soldiers out of the fight works just as well as killing them, and it has added benefits. The enemy must use resources to carry away and treat injured soldiers, and our own army can carry more bullets to put more enemy soldiers out of commission since the bullets are smaller and lighter than more lethal bullets.
The projectiles of the .222 and .223 calibers are of practically the same size as the projectile used in the little .22 rimfire bullet used for hunting rabbits and squirrels. The only way they are able to do significant damage to an enemy soldier or criminal assailant is that the bullet is pointed and is given enough speed to pass entirely through the body.
A few of the semiautomatic "AK" copy rifles called "assault rifles" by laws and gun control advocates use .30 carbine bullets. These have projectiles that are half again as big in diameter, and weigh about twice the weight of the .223. But the muzzle energy of this bullet is less than 1000 foot-pounds (less than the .222 and .223). Also, the projectiles of this caliber are rounded on the front, not pointed. So these projectiles generally go into a body and stop without going on through. The US military stopped using this caliber almost 50 years ago. It was the caliber of the M1 Carbine rifle.
A few of the semiautomatic rifles called "assault rifles" by laws and gun control advocates used .308 caliber bullets, which were used in WW1 and WW2 by the British. This is the only "assault weapon" caliber roughly equivalent in energy to a big-game hunting bullet, with a projectile weight of over three times that of the .223 and with a muzzle energy of about 2720 foot- pounds when fired from a test barrel.
If one of these (jacketed) bullets hits a person, it probably sends the person flying back, crashes through bone, turns a lot of flesh to mush, and still exits out the rear. While this caliber was being used by some militaries over 50 years ago, the US military was using the .30-06 that became one of the most popular big-game calibers (using soft-nosed projectiles). The only firearms of this category in the 1994 federal "assault weapon" ban are no longer produced. We don't know if any other semiautomatics are produced in this caliber. If any are, it's a safe bet somebody has banned them or is trying to.
The pistols in the so-called "assault weapons" category are typically like "UZIs" in 9mm, .41 or .45 caliber with muzzle energies of 345, 515-1050, or 370 foot-pounds. Bullets of these calibers typically do not pass through bodies. The .41 and .45 caliber bullets knock over a person and make large holes that cause heavy bleading and damage to any organs they encounter. The shock of the projectile hitting a person is very likely to knock the person out.
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