Machine gun bleg:

How many machine guns are currently in civilian (non-government hands) pursuant to the registration and tax system of the 1934 National Firearms Act? Of course these would be guns manufactured before May 19, 1986, thanks to 18 USC 922(o). How many NFA civilian machine guns have been used in a crime? My guess is about "100,000 guns in civilian hands" and "1 crime gun." However, comments with precise citations would be much appreciated.