In America, there are an estimated 250-280 million firearms. Out of these, it is of course impossible to tell how many are illegal weapons, as the very nature of a weapon being illegal would preclude its registration to the current owner or wielder.
But it is still fairly clear with a cursory glance of some statistics that illegal weapons have made their ways into the hands of plenty of criminals. 86% of juveniles in correctional facilities are reported to have owned a gun at some point, all of which would have been illegal weapons for the juveniles to own. 65% of juvenile offenders tend to own three or more illegal weapons and firearms.
In Rochester, New York, there was even the reported statistic that 22% of all young males in the city, as opposed to just those who are juvenile offenders, have carried an illegal gun for some period of time. While these particular statistics are oriented around youth, it still remains fairly clear that illegal guns are very prevalent, for these young people to have such easy access to them.
According to gunfacts.info, there were approximately 100,000 people convicted of "unlicensed operation of a motor vehicle each year." The point of this number is to prove that simply because something is licensed does not mean that unlicensed individuals will actually obey the regulations.
This is something of a logical fallacy, as the same rules do not necessarily apply to illegal weapons usage; but, according to all information, it would seem the analogy is apt, not least because unlicensed, illegal weapons will never be discovered unless the wielder is caught with the weapon.
Handguns are easily concealable, and as most illegal weapons are handguns, most illegal weapons will remain undiscovered. Furthermore, interestingly, gunfacts.info points out that criminals who may have committed a crime with a weapon do not actually need to obtain licenses or register their weapons, as this would be an act of self-incrimination, a ruling upheld in 1968 in the case of Haynes vs. the US.
Another study showed that five out of six gun-possessing felons did not purchase a handgun or otherwise get one through legal means, but instead procured an illegal weapon through the secondary market, or by theft.
The information of this study strongly supports the fact that handguns used by criminals are most often stolen or traded between each other, and therefore become nigh impossible to track in any meaningful fashion. All of these would be deemed illegal weapons.
According to a study conducted in 1997, which admittedly could be out of date but is one of the most recently conducted studies of this comprehensive nature , only 15% of firearms possessed by Federal inmates were obtained through a retail store. The largest portion of illegal weapons were given to the inmates by a family member or a friend, with the next largest portion having been given to the inmates by a drug dealer.
The bottom line of most of this information is quite clear: the firearms being used in crimes are overwhelmingly illegal weapons, and unfortunately, the government is unable to track illegal weapons because of their illegal, unregistered status.