The AR-15 was (originally) identical to the M-16, with the exception of one slight modification which renders it semi-automatic. In older versions of the rifle, you could simply buy a piece of metal and install it in the gun and it was once again fully automatic and essentially a M-16. It was a $50 part before 1986. In recent years they have modified the production of the AR-15 to make it more difficult to modify it to be fully automatic. But for years you simply added a part. You can still add this part for around $20,000 and spend a year or two going through licensing hoops.
I'd think of the AR-15 as a NASCAR car with a restrictor plate.