Fully automatic weapons have been illegal for over 30 years.  The gunfire that erupted at the horrific Las Vegas shooting massacre sounded faster than usual.  Many people, like myself, believed he had a fully automatic weapon.  What it very well could be in fact, is a bump fire stock.  The stock uses the guns recoil to assist it in firing rounds faster.  It is currently a legal device.

This YouTube user Jeff3230 demonstrates how a bump fire stock works.