Unlike other pro-gun people, I personally don't think there would be a civil war if guns were banned, at least not anything major. However, It would still be terribly ineffective, potentially even making problems.
The are more firearms than there are people in the U.S. and there is a strong following behind it. What would happen if something with a strong following and was already in circulation got banned? We do not need a theoretical answer for that: History had already proven it. Through 1920-1933 alcohol was banned throughout the US. The wide spread love of alcohol resulted in black markets appearing everywhere, so much so that many police forces couldn't deal with handling the sheer amount of illegal businesses. Mobsters took advantage of these illegal businesses, and organized crime skyrocketed.
I believe something similar would happen if we tried to ban guns. Crude, automatic SMGs can be built using parts in a hardware store, and with the advent of 3D printing more advanced guns can be home made. Printers that are capable of printing metal are especially capable of this, being able to build an entire M911 out off blocks of metal (and maybe plastic for the grip).
What do you guys think?