Take a look at this “drop-in sear“.
Still don’t get it?
What was once a $50 little metal gadget is now worth around $20,000 due to the 1986 ban on newly manufactured machine guns. This little item allows a regular semi-automatic AR-15 rifle (with a couple of M-16 parts thrown in) shoot fully automatic.
This one is fully registered and has a serial number, making it completely legal (and expensive) to own.
So if you have a gang load of money and want the fun of fully automatic rifle, start shopping for your own registered drop-in auto sear.
Sources: Machine Gun Mike, Eric Nestor