Forget the so-called "gun show loophole." These days, many gun-control advocates are far more focused on the large number of firearms sales that take place over the Internet, my colleague Philip Rucker reports.

A new study by Third Way, a centrist Democratic group, looked at gun ads posted on Armslist.com in ten states over several random days this summer. The authors found that more than 15,000 guns were on sale at any given time. What's more, they counted 1,928 ads "from prospective buyers asking to buy specifically from private sellers (thereby ensuring that no background checks is required)."

Groups like Mayors Against Illegal Guns have long argued that firearms sales facilitated by the Internet enable some illicit buyers to avoid background checks — which are designed to keep guns out of the hands of convicted felons or the mentally ill. But how do these online sales work, exactly? And what sort of regulations have been proposed?

Here's a quick recap of how things work now. There are three basic ways online sales can go down:

1) First, if you try to buy a firearm over the Internet from one of the nation's 130,000 or so* federal firearms license holders, then you have to go through a background check, period. They'll ship the gun to your nearest licensed dealer. There's no loophole there.

2) Likewise, if you want to buy a gun online from any seller in another state, they can't just mail the gun to your doorstep. The Gun Control Act of 1968 strictly regulates direct mail of virtually all firearms across state lines, save for antiques. Again, the seller has to ship the gun to a federally licensed dealer. The buyer would then have to go to that dealer, fill out paperwork, and undergo a background check before picking up the gun.

3) But there's also a third option. I can check out an ad posted online by a private seller and then meet up to buy the gun in person. (There may also be some situations in which the gun could be mailed within the same state, though not handguns.) That would be a private sale and federal law wouldn't require a background check — although some states would. California and Rhode Island require background checks for all private sales, while 12 states mandate checks for private handgun sales.

That last option is basically the same thing as if I had responded to a classified ad or a posting on an office bulletin board for a gun, except that the Internet makes these private transactions somewhat easier. At last count back in 2000, the Justice Department found about 4,000 websites offering guns for sale. Presumably the number has risen since.

Now, it's still a felony for any private seller to knowingly sell a gun to someone who likely couldn't pass a background check because of their criminal record or history of mental illness. But this is a hard law to enforce — and it does seem to get flouted at times. One undercover investigation by New York City in 2011 discovered that 54 percent of dealers contacted on Armslist.com were willing to sell their guns even when the researchers explicitly stated that they "probably couldn't pass a background check."

No one really knows, however, how many guns bought online in the United States are acquired illegally or used in crimes. The Bureau of Alcohol, Tobacco, Firearms and Explosives doesn't track this. The crime data are basically anecdotal — for example, Stephanie Mencimer of Mother Jones reported in February that "Weapons obtained through [Armslist.com] have been tied to the murders of four people and one suicide." That's not much to go on.

So what about regulations? Earlier this year, the Manchin-Toomey gun bill in the Senate proposed to extend federal background check rules to all sales facilitated by the Internet — not just interstate sales. If I saw an ad online and went to go buy a gun from a private seller, then we'd both be required to go to a federally licensed dealer, who would conduct the background check and complete the transaction.



Sens. Joe Manchin (D-W.V.) and Pat Toomey (R-Penn.), of the famed Manchin-Toomey gun bill. (Melina Mara/THE WASHINGTON POST)

In practice, gun experts note, that rule might prove tricky to enforce. After all, under Manchin-Toomey, most private gun sales would remain unregulated — save for those facilitated by the Internet. But how would officials prove that a given sale was initiated by an online ad? Maybe the buyer heard about the gun through word of mouth, or the classifieds. What's more, the seller wouldn't need to keep records of the sale.

(Granted, there might be other ways to enforce such a law: As the University of Chicago's Jens Ludwig explained here, undercover police officers doing gun buys could conceivably help crack down on illicit sales.)

Other opponents of the Manchin-Toomey measure, like Sen. Jeff Flake (R-Ariz.), said the regulation was simply too intrusive. "It would expand background checks far beyond commercial sales to include almost all private transfers — including between friends and neighbors — if the posting or display of the ad for a firearm was made public," Flake wrote back in April. "This simply goes too far."

In any case, the Manchin-Toomey bill failed to get 60 votes in the Senate, so new federal regulations for online gun sales are a moot issue for now. But Senate Majority Leader Harry Reid recently said that Congress was "almost certain" to revisit background checks in 2014. The issue could well come up again.

* Correction: This post originally misstated the number of federal firearms license holders in the United States.