Crimes involving firearms often happen due to guns that make their way into the hands of people who should never own one. While laws try to prevent this, the black market overrides the system to get weapons on the streets.
The Giffords Law Center describes gun trafficking as the process by which guns end up on the black market.
How it happens
Trafficking often happens when guns move from one state to another because of relaxed laws in one of the states. For example, if state A makes it easy to buy a gun, then someone from state B may buy a bunch of guns in state A, take them to state B and sell them to other people who cannot get a gun legally in state B.
Trafficking can happen in a few different ways. Sellers who do not follow the law are a huge contributor. Another common way this happens is through straw purchases, which are when someone buys a firearm for someone else who cannot legally buy one themselves. Bulk purchases are another prominent way these weapons get into circulation, but they also can come from the theft of legally purchased items.
The biggest reason why gun trafficking is a serious issue is that it allows guns to get into the hands of criminals. Specifically, it supersedes the laws in states with strict gun laws to allow those who should not own a firearm to get one easily. The proof is in the fact that many people convicted of criminal charges use black market weapons in the commission of crimes.