an amendment to the Firearms Owner Protection Act, which Reagan signed as it was overall very positive in restoring firearms rights, changed the federal laws pertaining to fully automatic firearms. Anything made after that time, if fully automatic, could not be legally owned by a non-military or police agency. This has created an artificial shortage of machine guns in the USA and caused the price of an M16, for example, to decouple from the price of an AR15. They used to be within a few dollars of one another.
A new AR15 can be purchased for $1,000 or less. The newest M16 you can legally buy is 27 years old and currently sells for more than $20,0000. That's not a typo.