Go ahead.  Ban the sale of assault rifles.  There are currently around 2 million assault rifles in the US.  And it only takes one to kill 50 people.  Do you think an Islamist extremist will have a problem getting his or her hands on one out of 2 million assault rifles in the US?  Banning assault rifles is a feel-good ideology and a small inconvenience for bad people.  It won't stop the real problem - hatred from extremists and psychos that want to kill.