The problem with most gun control laws is that they do not address the root cause of violent crime. The root cause is not the existence of weapons. Weapons have existed as long as humans have. The root cause is the huge imbalance in combat power between someone with a gun and his intended victim. The economic principles of cost verses benefit also affect the criminal mind. The cost is his probability of getting arrested or hurt and the difficulty of committing the crime. The benefit is whatever he plans to take from his victim. You can see this documented in a study done by John Lott. His study is the most complete study on crime reduction that has been done to date. You may also find this argument against gun control instructive. It contains significant amounts of historical information.
If a Gun Control Law reduces the cost to the criminal, then crime will go up. If gun control increases the cost, then crime will go down. The history of gun control shows that most of the time, it just makes it easier for evil people to commit their crimes. You can see this fact in this documentary on The History of Gun Control: Innocents Betrayed.
Most gun control actually reduces the cost to the criminal of committing a crime. It may get slightly harder for the criminal to obtain a gun, but at the same time the law makes it more difficult for the victim to defend himself. Since the criminal can avoid the law, but a law abiding citizen cannot, the difficulty added by most gun control laws is heavily in the criminals favor.
Concealed Carry laws work effectively to reduce crime by increasing the cost of committing the crime to the criminal. An example of arming the citizens to reduce crime can be seen in the city of Kennesaw, Georgia. This city required its citizens to own a gun and saw a dramatic drop in crime rates after passing the law. The city saw crime rates drop 89%, and currently has the lowest crime rates of any city of comparable size in the country. More evidence that citizens using firearms to protect themselves can save lives can be seen in this video.
Regarding the current push for an assault weapons ban, and stricter control of gun show sales, we can see in history that the previous assault weapon ban did absolutely nothing to reduce crime or violence, and that only 1.9% of guns used in crime come from gun shows. These figures are documented by the US government, and are cited in the US Senate here. The reason that the assault weapons ban does nothing to reduce crime is easy to understand. The simple fact is that assault rifles are already banned, and have been since 1934. Regulations on fully automatic weapons were strengthened in 1968 and again in 1986. It is today not legal to purchase a real assault rifle such as the military uses. So called assault weapons bans are only banning certain cosmetic features that make a normal hunting rifle look like a military rifle. Cosmetic features such as color or the type of stock have nothing to do with making a rifle more dangerous, and that is why banning such things does not make the country any safer.