As someone mentioned, the weapon that we're hearing in this attack is a gun that was modified to shoot at near automatic speeds. The AR-15 you see is a true automatic with a beta-magazine attachment.I'm not very educated on guns but that AR-15 sounds like it's definitely firing a lot faster than the other video. Is this something that can change easily?
It is likely that the person just modified the trigger to make it cycle faster and applied a larger magazine. The only thing I could think of that is easy enough to acquire is an AR-15 with a large capacity magazine.