"We banned machine guns in the 20's."  

We did no such thing!  Fill out the ATF forms, get the FBI background check, pay your $200 tax per machine gun and you can own one, too!

As of February of 2016, there were 490,664 legal machine guns in private hands according to the FOIA letter from ATF.

We don't yet know if the apparently automatic weapon(s) used in this massacre were legal or illegal.  

Either way, this is the first mass killing in the US to use an actual automatic weapon that I have seen reported.  

http://www.nfatca.org/pubs/MG_Count_FOIA_2016.pdf