One quick item: A bullet is the slug that actually moves down the barrel, comes out the end, and delivers a godawful mess of kinetic force to whatever it hits. The shiny brass thing that contains the bullet, the explosives that drive the bullet, and the mechanism that sets it off is a cartridge, or "shell." For convenience sake, I'm sometimes going to say "bullet" when talking about the whole cartridge. Purists beware.
Automatic, Semi-Automatic and Other
The bit of information most likely to be mangled in any reporting on a mass shooting involves the action of a gun. That is, once the gun has been fired, what action does it take to make it fire again?
Many older guns require manual intervention to eject the spent cartridge, load a fresh cartridge, and make the gun ready for firing. Rifles, including military rifles made before the 1950s, are often bolt-action. They work by pulling back on small handle that's attached to a long "bolt." The gun fires, the shooter raises and pulls back the handle, causing the old cartridge to eject, he then shoves the handle forward to bring a new cartridge into place and brings the handle down for firing. A skilled operator can do all this very quickly. Likewise, many shotguns use a pump action. You've probably seen this in movies where Our Hero is going up against a vampire, terminator, or similarly tough beast. Pulling back the handgrip sends a spent shell flying, while pulling it forward again readies the gun for a new shot. It also makes a very cinematic series of clicks.
Automatic and semi-automatic weapons don't require help in tossing out the old shell and loading up the new. They get their energy from the firing of the cartridge, capturing the energy or gases of the spent shell to bring the next cartridge into position. The difference between a fully automatic weapon and one that's semi-automatic is simple: A fully automatic weapon begins firing when the trigger is pulled and keeps firing until you let off the trigger (or run out of bullets), a semi-automatic weapon fires once for each pull of the trigger.
How quickly you can fire a semi-automatic weapon depends partly on the design of the gun, partly on the speed of your reactions. Most of the time, the answer is Very Damn Fast. As in multiple shots in a second. That's unlikely in a real-world situation, but with a semi-automatic the next shot is there when you're ready. How fast you can move your finger is generally the biggest limiting factor.
Fully automatic weapons (which most people tend to think of as "machine guns", though the Army reserves that term for larger weapons) are not legal for private citizens in most cases. You may see fully automatic weapons available to test at a gun range, or in use at special events. But you will rarely see one at all. None of the mass shootings in the United States within recent decades has involved a fully automatic weapon. They are regulated, and that regulation appears to be working.
Semi-automatic weapons are extremely common. Yes, these weapons have been used in many mass shootings, but they are also an increasingly popular type of rifle for hunting. Semi-automatics have also become a very popular form of shotgun. And of handgun (revolvers are not semi-automatic because the mechanical motion of the trigger positions the next shot).
Some semi-automatic weapons are based directly off fully automatic military models. These guns may have cosmetic differences with their military relatives, but in the same way that a Cadillac and a Chevy may be the same under the sheet metal, they share the same bones as the military guns. They are not fully automatic, and (for the last couple of decades, at least) it has been extremely difficult to convert them to become fully automatic. However, these weapons share many other characteristics with their military cousins. In the United States, there are many variations on a rifle called the AR-15, which is a semi-automatic version of the military M16. Many variations as in dozens, from several different manufacturers. And the AR-15 is just one category. There are semi-automatics that descend from the ever popular AK-47, the Chinese QBZ-95 and several more. These weapons are often able to accept accessories that were originally designed for the military version, or to accept modified versions of those accessories. This includes tripods, extended magazines, laser sights, night vision scopes, and all manner of ridiculous extras that those fearing the looming economic / racial / zombie apocalypse can bolt on to make their guns look meaner and kill more readily. It's these military-derived semi-automatic rifles that are most commonly called "Assault Rifles."
What can we do here? Eliminating or restricting all semi-automatic weapons may seem like the most obvious choice, but it would also be extremely unpopular with hunters, with target shooters, and with gun owners in general. These days, semi-automatic isn't just the first thought of anyone going in to buy a rifle or shotgun, it's almost the only thought. Legislation aimed at all semi-automatic weapons would be difficult to pass, no matter how strong the political tailwind. What can be passed is severe restrictions on semi-automatic weapons based off military models. Nearly half the killers involved in mass shootings over the last 30 years have carried assault rifles. They should be the clear target of any legislation.
The weapon carried in most mass shootings? Semi-automatic handguns. Often more than one. Banning those would be a much tougher fight.
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There are three common systems for measuring caliber. When you see someone talking about a .223 or a .44, the caliber is in inches. A .22 rifle fires a bullet that's 0.22 inches in diameter. 9mm or 10mm is just what it sounds like—a gun chambered to fire a bullet 9mm or 10mm in diameter. If you remember your metric conversions, you can do the metal gymnastics to swap them around. A .44 caliber bullet is a bit larger than a 10mm. A .357 and a 9mm are close enough that some guns will fire either size.
If you see the term "gauge," you know the weapon in question is a shotgun. Shotguns can fire a single bullet (usually called a "rifled slug") but most of the time a shotgun fires a number of small metal pellets. Shotgun gauge is ... complicated and the numbers we commonly use mix up two different ways of calculating the size. Just remember that in general, the smaller the number, the larger the shotgun diameter. So a 12 gauge is bigger than a 20 gauge, and a 20 is bigger than a .410. Those three are the only shotgun sizes you're likely to encounter, so it's not too hard to keep straight.
As you might expect, if all things are equal, a larger bullet carries much more impact. However, all things are almost never equal. Speed, bullet design, cartridge design, the type of gun used, they all make a big difference.
Take a look at the picture again.
The first three cartridges on the left contain bullets sized at 9 mm, .40 and .45. All are designed for handguns, and all of them deliver devastating impact. Next to them is a 5.7mm. Compared to the big lugs on the left, the 5.7 (the skinny, pointy one) might seem like a pipsqueak, but that long cartridge below the small bullet helps to give it a very high velocity. It's a bullet that was specifically designed as a military anti-personnel round, designed to offer more "terminal performance" than the 9mm round it replaced. That's military speak for "more likely to kill whoever it hits." The 5.72 round is also designed to penetrate body armor. It's used in militaries around the world, and by the U. S. Secret Service. It's also available at your local store.
When it comes to bullets, there is no "safe" caliber, and smaller does not always mean less deadly. There are bullets designed to tumble so that they rip paths through flesh, bullets designed to shatter on impact, bullets that can cut not just through body armor, but through walls. High penetrating rounds, made from hardened metals, don't only pass through walls or windshields, they also pass through people, so that one bullet can strike more than one victim. In addition to terminal performance, you'll also see terms like ballistic trauma and hydrostatic shock. They're all measures of a bullet's ability to royally f*ck you up in different ways.
Cartridges designed for a rifle have the same range of variability. The smaller of the two rifle cartridges above is a 5.56mm while the larger is .30 caliber. In fact, it's not just a .30 caliber, but a .30 caliber magnum meaning that the cartridge is proportionally larger than a standard .30 caliber and delivers more oomph. Cartridges of this size were used by many World War II era military weapons, and variants on the .30 are popular today with hunters. A modern .30-06 cartridge used in big game hunting actually delivers around 3,000 ft-lbs of energy, allowing hunters to take down a large animal at 300 yards or more.
Back to that little 5.56mm. It looks kind of puny next to the .30. Did I mention that it's also called a .223? That's the cartridge that's fired by the M16, AR-15 and the Bushmaster rifle used to such horrible effect at Sandy Hook Elementary. Where the .30 is a WWII era military round, this is a modern cartridge. The 5.56 is NATO's combat round. It's designed to penetrate deep into soft tissue. It delivers high hydrostatic shock causing damage even in parts of the body not struck by the bullet. The .223 Remington cartridge (which is likely what was used on the victims at Sandy Hook) actually delivers almost exactly the same amount of energy to target as a big .30 magnum and can be used in hunting, but a lot of hunters don't like it. It's unreliable an inaccurate at long range, and at short range it tends to tear things up. (Note: in comments, several people have defended the .223 as being a better round for hunting in brush).
What can we do here? Regulating ammunition by type would by no means be a cut and dried approach. Just cutting out bullets above or below a certain size wouldn't really work, and targeting specific cartridges would lead to a kind of whack-a-mole effort as new variants appeared. But it should be possible to restrict cartridges that are designed to penetrate body armor, those using hardened materials (like the so-called "cop-killer" bullets) and those pistol bullets designed to fragment on impact.
Magazines & Clips
While bolt and pump action guns generally store their ammunition directly in a chamber inside the gun, automatic and semi-automatic weapons use magazines or clips. These can be small and completely contained within the body of the gun (the clips of many semi-automatic pistols hide within the gun's grip) or they may curve down, up, or sideways to allow for higher capacity. The basic purpose of magazines is to allow for extended capacity and rapid reloading. On many guns, the magazine can be released quickly, and a new magazine put into place. A skilled user can switch magazines in a matter of seconds.
Many pistols have standard clips that contain between 10 and 15 cartridges. Semiautomatic rifles are more typically sold with larger magazines. The AR-15, one of the most common platforms, most often comes with a box magazine holding either 20 or 30 cartridges arrayed in column that curves down and slightly forward. Optional box magazines hold 40-50 cartridges.
However, this is just the start. Both drum and STANAG (a NATO design) magazines are available with capacities of 90-100 cartridges. These large magazines often make the weapon un-wieldy, but the STANAG variety are more compact than the older drums.
This is a Bushmaster Adaptive Combat Rifle equipped with an extended capacity STANAG magazine. Though it differs from what's been in the news, I suspect this arrangement is similar to what students and teachers saw last week.
What can we do here? This is simple: Make the sale of extended magazines illegal. In fact, we should reduce the size of standard magazines. Magazines are already available that limit the capacity of AR-15 and other military-derived rifles to 5-10 cartridges. Make those mandatory at time of sale.
Net result if we can pass legislation that will restrict the sale of semi-automatic rifles based off fully-automatic military designs, limit the availability of bullets that are designed for high mortality, and strictly reduce the size of magazines that can be used with semi-automatic weapons we'll have taken a good step. And this sensible step will not affect anyone's ability to hunt or defend themselves.