Earlier quoted context omitted.
Actually, I think assault rifles cannot be legally owned by civilians in the US. I'm not from, or living there so corrections are welcome but I remember that the Texas gunman had a special setup that allowed his weapons to fire in automatic mode, or something close to.
No. That is incorrect. Even fully automatic long-guns (think AK47, AR-15 with a full-auto mod) can be owned legally after paying for the license with the ATF.
People who buy them don't go on shooting sprees.