The truth is, is that this is America. It's relatively easy to get whatever you want if you know where to look - drugs, guns, illegal sex, etc. These gang shootings, all these shootings in the cities where it's teens killing people (which is the number one cause of gun violence now in multiple cities), many of the mass shootings, are committed with illegal guns. They either buy them on the street for a few hundred bucks or they slip through the cracks because people don't pay much attention, even when this person shouldn't legally be allowed to own a gun with the laws we currently have.
An example of the latter was Cho, the Virginia Tech killer, who killed over 30 people with a couple of handguns even though he wasn't supposed to legally have them because of his obvious severe mental disturbances. And then before that, most school shootings were committed by minors who aren't legally allowed to have the weapons in the first place, or were illegally acquired and modified, like with Columbine. I don't really see what gun control could've done to stop them. Street gangs in America have fully automatic weapons like AK-47s and other illegal weaponry, nevermind the petty AR-15s.