Updated, 5:35 p.m.

Suspected terrorists can't fly on planes, but they can buy guns. The feds can track sales of fertilizer, but not semi-automatic rifles. Brick-and-mortar gun dealers perform background checks, but online ones often don't. These are three of the many odd aspects of the gun trade that are now being reconsidered after the massacre of 20 children and six adults at the Sandy Hook Elementary School in Newtown, Connecticut.

Here are five potential steps that gun owners, gun vendors, manufacturers, law enforcement and legislators might consider to stem mass-casualty gun violence – without shredding the Second Amendment, and without forcing gun owners to give back their weapons. No one measure will eradicate such attacks: Perfect security is an illusion, and one easily used to snatch away people's liberties. None of the proposed fixes are foolproof. Each of them comes with the potential to seriously backfire. But after Sandy Hook, it's time to a take a fresh look at the state of America's firearms market.

A bullet casing showing identification codes, at center, is shown through a microscope at a news conference at the Los Angeles Police Academy Tuesday, Aug. 14, 2007. Los Angeles Mayor Antonio Villaraigosa and police officials endorsed a bill Tuesday that would make California the first state to require gun manufacturers to include this "microstamping," for all new semiautomatic handguns, a mechanism to help investigators match casings to guns. (AP Photo/Reed Saxon) A bullet identification code seen through a microscope at the Los Angeles Police Academy on Aug. 14, 2007. Photo: AP/Reed Saxon

Microstamping ————-

Imagine every semi-automatic gun – those that automatically reload after every trigger pull – carried its own unique signature, transferable to every bullet at the point of firing. That's what happens with an engraving technology called microstamping: Once engraved with a laser during manufacture, the gun's firing pin imprints a tiny alphanumeric code onto the bullet's shell casing and the primer used to fire.

Pro: Shell casings are more likely to be left at crime scenes than firearms or fingerprints are. "Stamp" the shell and you've added a layer of evidence about a perpetrator for police, one that's theoretically more exact than ballistics testing. It's primarily a method to mitigate gun violence after it occurs, but it's possible microstamping could have some deterrent effect as well.

Con: It's only a tool for semi-automatics, so it's irrelevant if you're reloading, say, your shotgun shells manually. You'd have to mandate microstamping at the point of manufacture for new guns, meaning it'll be irrelevant for the estimated 310 million guns already in use in the country. It's theoretically possible to file off the marking on the firearm pin, although practically speaking the engraving is invisible. Finally, the data on the shell casings can only identify the last legal owner of the gun.

Los Angeles Police chief Charlie Beck displays a handgun loaded with an extended clip Wednesday March 2, 2011 during a news conference in Los Angeles. Chief Beck endorsed a proposed federal ban on the sale of high-capacity ammunition magazines that have been used in mass shootings. (AP Photo/Nick Ut) A high-capacity pistol clip in the possession of the Los Angeles Police Department. Photo: AP/Nick Ut

Magazine Limitations ——————–

Jared Loughner never had to reload when he shot Rep. Gabrielle Giffords and 12 others in Tucson, Arizona, in 2011. His Glock carried a 33-round extended magazine; Loughner fired 31 bullets. The U.S. has limited magazine size before: The expired 1994 Assault Weapons Ban banned magazines carrying more than 10 rounds. (.pdf)

Pro: Anytime a shooter has to stop to reload increases the chance that victims could escape; that law enforcement or others can stop an assailant; and, basically, fewer people will die. Robert Wright of The Atlantic goes a step further and proposes a ban on firearms carrying more than six rounds or a detachable magazine, meaning a shooter would have to reload bullet by bullet.

Con: There isn't strong data correlating restrictions in magazine size with drops in gun crime. As the Washington Post's Brad Plumer points out, the assault weapons ban exempted about 30 million high-capacity magazines, so studying the impact of the ban is surrounded in statistical noise. A shooter can always carry multiple loaded weapons.

d∂wn /Flickr

Equalizing Online and Offline Gun Sales —————————————

If you want a gun to commit a crime, you should buy one over the internet. Federally licensed gun dealers need to conduct background checks on prospective buyers. But online, you can resell your guns in a burgeoning secondary market, on websites like ArmsList, without being a licensed dealer, and without background checks. While online vendors are supposed to ship their guns to a federally licensed dealer who'll perform the background check, a 2011 New York City investigation found that's not always the case in practice. (.pdf) The rules vary site to site, but many sites take the eBay or Craigslist approach of staying hands-off after visitors sign a term-of-service agreement. The 2007 Virginia Tech shooter, Seung-Hui Cho, bought his guns online; so did the Aurora shooter.

Pro: You'll shut down an easy path for people to acquire dangerous weapons without answering questions. The changes to online gun marketplaces, the New York City investigation suggested, are feasible without shutting down the resale markets themselves: either authorized gun dealers or law enforcement would perform the background checks, or the sellers would have to verify a buyer's valid gun permit – something the investigation judged to be "relatively easy."

Con: It'll require a lot of enforcement. Imagine having a regulator reviewing every eBay auction. Since online gun stores are basically connector points between consumer and vendor, it's easy to imagine illicit transactions moving to a different forum – i.e., if you reach me over ArmsList and offer me big money for one of my guns fast, maybe I'd rather do business with you in a less conspicuous forum, like a vacant lot.

Inha Leex Hale /Flickr

Put Gun Registries in Terrorism Databases —————————————–

If you're a suspected terrorist, you'll set off all kinds of alarm bells if you try to buy the precursor materials for a bomb. But if you going on a firearms shopping spree, you're in the clear, since the government can't legally maintain a database of gun owners. In other words, "there is no basis to automatically prohibit a person from possessing firearms or explosives because they appear on the terrorist watchlist," a 2011 Government Accountability Office report found. (.pdf) Read that again: suspected terrorists can buy all the guns they want. Perhaps that shouldn't be so.

Pro: The last people who should have guns are suspected terrorists, right? At the very least, law enforcement needs tools to be able to track the prospective weapons purchases of people they're monitoring out of fear they'll commit an act of terrorism, especially since it's so easy to buy guns.

Con: The U.S. government often mislabels ordinary citizens as terrorists-in-training – which makes terror watchlists awfully problematic. They contain the names of people who've never committed and won't commit any crime, sometimes because of incorrect transliterations of their names, as a Department of Homeland Security study found. (.pdf) An 8-year-old boy was once on the government's "selectee" list for extra screening at airports. And once you've been placed on a watchlist like the "no-fly" list, there's no obvious mechanism for getting off it: The government doesn't have to tell you you're on it.

From left: Queens District Attorney Richard Brown, New York City Police Commissioner Raymond Kelly, and New York State Senator Malcolm Smith with guns collected through a buyback program in August, 2012. Photo: Queens District Attorney's Office From left: Queens District Attorney Richard Brown, New York City Police Commissioner Raymond Kelly, and New York State Sen. Malcolm Smith with firearms collected through a gun buyback program in August 2012. Photo: Queens District Attorney's Office

Cash for Guns ————-

This one isn't a technological solution at all; it's an economic one. Police departments across the country offer cash for guns. Australia has experience with it at the national level: After a mass shooting in 1996, it bought back nearly a fifth of all shotguns, handguns and rifles in private use, some 600,000 of them.

Pro: There hasn't been a mass casualty incident in Australia since 1996. An Australian study that the Washington Post's Dylan Matthews found estimates that the law led to a 59 percent decline in the firearm homicide rate and a 79 percent decline in the firearm suicide rate.

Con: It'll be expensive. A recent congressional study found that the U.S. has over 300 million handguns, rifles and shotguns, which is about one weapon per American. That shows a robust demand for firearms in the United States that may either render buyback programs marginal or risk stressing state and federal budgets.

Again, none of this is to say that any of these measures, individually or in concert, would necessarily prevent another Sandy Hook. There will always be psychopaths who figure out ways to kill people. But it is to say that there are gun-control options that either make it harder to pull off a mass-casualty shooting or can mitigate its effects, short of the unrealistic demand that Americans surrender their hundreds of millions of guns. If we're willing to discuss them, that is.