Guns come from all across the world. Specifically places that don't regulate like the USA does. Come on now, hopefully you just don't know better. There are enough weapons to supply the entire USA with about 1-2 weapons per person. Take the AK-47 for example. There is just an AMAZING surplus of AK-47's available. Some army/military and some for civilian use.
Take a quick look at these numbers,
http://en.wikipedia.org/wiki/Arms_industry.. the USA is a top contender, but guns come from across the entire world. Regulating USA won't slow any of the black market down.
In 2007 they estimated around
870 million firearms.. and that is 2007. (this was a quick google search so I could give real numbers) Guns lifespan, in the responsible owners hand, can last generations. I still have a few weapons from WWI that my family has passed down. They still fire. Ammo hard time come by, but they fire just as well.
Sorry to tell you, but deterring people from buying over the counter weapons only affects the legit firearm owners. Believe it or not, USA does have pretty good deterrents already in place. Ex; Firearm Dealers call in every single sale, recording SSN, and serial number. If the Fed's respond with a "RED - NO GO" response -- the sherif and police could be called. You are not able to purchase with any type of felony on your record.
"Bad guys" get their stock from the black market or even theft. The theft part, is where I must completely agree. If a weapon is used in a murder and was stolen, that sucks because it could have been prevented by not allowing someone to buy without a proper safe installed.