Most drug addicts can't manufacture their own drugs, but they always manage to get them.
Building guns by hand in a garage can be done. It's common in the Philippines. Danao City is nicknamed gun city.
http://www.youtube.com/watch?v=lLApVGIU8eQ Interestingly enough, the popularity of guns in America, has cause legal gun manufacturers in the Philippines to start hiring these garage gunsmiths to keep up with demand. So legal sales are taking away from the black market there.
In a modern machine shop with CNC machines it's child's play. The most popular gun in America, the 1911 is over a 100 years old. It's not high tech, anyone with a little knowledge and basic machinery could make one.
Then there's our porous border with Mexico. Already billions of dollars worth of illegal drugs cross our borders every year. It wouldn't be hard to add guns to that illegal distribution rout.
Then there's the international black market. They'll sell to anyone. They already sell to the Mexican drug cartels.
The Chinese will sell to anyone. A major Chinese arms manufacturer, Norinco, was caught trying to sell fully automatic weapons to street gangs and drug rings in 1994. They've been banned from selling guns in the US, but if there was a gun ban, you can bet their guns would end up here.
Mexico has tough gun control laws, and their crime rate is worse than ours.
There are over 250 million guns in America. Banning them, wouldn't make them magically disappear. The genie is already out of the bottle; it's to late to try to put it back in.