The so-called "gun show loophole" is one of the most prominent talking points of those who would restrict firearm ownership. According to that side, it is possible to go to a gun show and purchase any firearm from anybody, even a fully-automatic suppressed short-barreled rifle that, without a $200 tax stamp and an entire bookshelf's worth of paperwork, is several different violations of the National Firearms Act rolled into one, and therefore leads to several years in prison. However, this is not the case.
In actuality, although it is true that half of all transactions made at gun shows are without background checks, an often cited statistic, the vast majority of those are not firearm sales, but knife sales, T-shirt sales, ammunition sales, and the like. It is true that 10-15% of firearm sellers at gun shows are not by holders of Federal Firearms Licenses, and therefore, not subject to background check laws. However, due to the laws regulating the firearm trade, it would be a felony for them to make more than a few sales in a short period of time, leading to private sales being proportionally far less common than sales from FFL's at gun shows. Private sales are only allowed for in-state transactions, so it is a felony to cross state lines and make a private transaction. This is because of the Commerce Clause of the US Constitution: the private seller is assumed to have bought his weapon in state, and now that he no longer has any use of it, he's going to transfer ownership to somebody else in the same state, so the federal government has no authority over the transaction.
The calls for background checks to be required for all firearm transfers would be much less idiotic if the people making those calls didn't fight for it to remain illegal for those who don't hold Federal Firearms Licenses to contact the National Instant Check System.
Another, more recent so-called loophole to become a talking point is the "internet sales loophole." Apparently, it's possible to order firearms online, and have them delivered directly to your house without being background checks. There have been deliberate attempts to equate private sales of questionable legality over Craigslist or a similar website with sales from websites that specialize in firearm sales, and often, use that to say that the Virginia Tech shooter, who purchased his weapons from one such website, was able to buy weapons without a background check. That statement could not be farther from the truth. In order to purchase a firearm from such a website, it has to be delivered to a nearby Federal Firearms Licensee (usually a gun store), who then submits your information to the FBI, and after the background check is finished, you get the weapon that you ordered online. In fact, there is no way to get a firearm delivered through US Mail except between two Federal Firearms Licensees or if somebody sends a weapon to a licensed gunsmith for repairs or modification.