One of the methods that I am aware of, and it's rather simple, is via TOR. You can get to the Silk Road, and purchase illegal firearms with Bitcoins. This method is pretty common, and easy for a criminal who can't get their firearms by legal means.
You're wrong there, not only is using SR for firearms not common its extremely rare and highly unlikely to see the product actually delivered to you without a warrant for your arrest. The common method of acquiring firearms illicitly is listed below.
There is also no inherent thing as an "illegal firearm" only a firearm device that a government may or may not have some regulation thereof, I'm afraid the Brady campaign and Hillary Clinton has clouded the language you use to describe this issue. Firearms are firearms, neither legal or illegal. Kind of like saying "illegal cars," illegals guns is a facetious statement used by the anti-gun lobby, if it is a car or not, the legal status has nothing to with how they are acquired. Buying it over SR might be illegal (then again, maybe not if they go through their FFL, how do you know?). Firearms are shipped by mail to FFL licens holders all the time and prior to the 1960's mere citizens could mail order firearms to their door.
What is common is mister "thug for life with a felony" gets his girlfriend to buy the guns for him.
WRT to OP, I want you to make illegal the physical process of projecting a piece of lead propelled at great force down a metal tube. After you have altered the laws of physics to make this happen at that time you might be able to make firearms "illegal" in the only meaningful way possible. Until that time mankind will manufacturer and create firearms to varying degrees of sophistication with the most basic of metal shop machinery and a little bit of chemistry.