The Firearms covered under the National Firearms Act (NFA).
Fewer than ten registered machine guns (out of over 240,000 in the nation) have ever been used in any type of crime (including nonviolent offenses such as failing to notify ATF of address changes, etc.). The criminal use of other legally-owned NFA weapons (e.g., silencers) is similarly rare. The Title II weapons used in prominent crimes, such the AK-47s used in the North Hollywood shootout of 1997, have universally been illegally-owned or illegally-converted weapons.
Maybe it's because these items are strictly regulated:
All NFA items must be registered with the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF). Private owners wishing to purchase an NFA item must obtain approval from the ATF, obtain a signature from the Chief Law Enforcement Officer (CLEO) who is the county sheriff or city or town chief of police (not necessarily permission), pass an extensive background check to include submitting a photograph and fingerprints, fully register the firearm, receive ATF written permission before moving the firearm across state lines, and pay a tax. The request to transfer ownership of an NFA item is made on an ATF Form 4. Many times law enforcement officers will not sign the NFA documents. There have been several unfavorable lawsuits where plaintiffs have been denied NFA approval for a transfer. These lawsuit include; Lomont v. O'Neill 2002 9th circuit, Westfall v. Miller 1996 5th circuit, and Steele v. National Firearms Branch 1985 11th circuit. In response Tennessee and Alaska have passed state laws which require the CLEO to execute the NFA documents. On October 28, 2010 in response to a writ of mandamus a Tennessee Williamson County Chancellor Robbie Beal found that the sheriff or CLEO is not required to execute NFA documents according to Tenn. Code Ann. 39-17-1361.
The NFA makes certain conduct a criminal offense, in relation to engaging in business as a manufacturer, importer, or dealer with respect to (NFA) firearms without having registered or paid a Special Occupational Tax (SOT); receiving or possessing a firearm transferred to oneself in violation of the NFA; receiving or possessing a firearm made in violation of the NFA; receiving or possessing a firearm not registered to oneself in the National Firearms Registration and Transfer Record; transferring or making a firearm in violation of the NFA; or obliterating, removing, changing, or altering the serial number of the firearm.
Violations of the Act are punishable by up to 10 years in federal prison and forfeiture of all devices or firearms in violation, and the individual's right to own or possess firearms in the future. The Act provides for a penalty of $10,000 for certain violations. A willful attempt to evade or defeat a tax imposed by the Act is a felony punishable by up to five years in prison and a $100,000 fine ($500,000 in the case of a corporation or trust), under the general tax evasion statute. For an individual, the felony fine of $100,000 for tax evasion could be increased to $250,000
In 1984, then-ATF Director Stephen E. Higgins testified before the House Committee on the Judiciary, Subcommittee on Crime, that registered NFA firearms aren't likely to be used in crimes, and more specifically that registered machine guns aren't considered a significant law enforcement problem. "In summary," Mr. Higgins concluded, "I would say that the National Firearms Act provides a satisfactory regulatory framework for keeping track of legally obtained weapons possessed by responsible, law-abiding gun owners."
So, the framework ensures that these weapons are well regulated and kept in the hands of people who are known to be law abiding. There are also very strong incentives to keep the people who own them law abiding through stiff penalties should they fail to remain within the law.
While the people who advocate removing the strict regulation for these weapons fail to recognise is that it is that regulatory framework which keeps them from being as abused as other firearms. We have already seen them being subject to abuse prior to the NFA, which was also the reason for the NFA (does the Saint Valentine's Day Massacre ring a bell for you?).
If anything, the fact that NFA weapons are so rarely seen in criminal activity shows that gun control works. It present an argument that stricter regulation of other firearms might have the same result in reducing crime.
Remember Heller-McDonald found that laws imposing conditions and qualifications on the commercial sale of arms did not violate the Second Amendment. (Heller at 54-5)
Also, don't forget that Alan Gura said during the oral argument:
So, you shouldn't expect for NFA firearms to be deregulated, but it would make more sense and is backed up by the NFA experience that the act should be extended to other firearms.
MR. GURA: Well, my response is that the government can ban arms that are not appropriate for civilian use. There is no question of that.
JUSTICE KENNEDY: That are not appropriate to –
MR. GURA: That are not appropriate to civilian use.
JUSTICE GINSBURG: For example?
MR. GURA: For example, I think machine guns: It’s difficult to imagine a construction of Miller, or a construction of the lower court’s opinion, that would sanction machine guns or the plastic, undetectable handguns that the Solicitor General spoke of.