There is also a type of registered sear that is used in standard semi-auto AR-15 type weapons. These are commonly called DIAS (Drop In Auto Sear) and Lightning Links...
The DIAS is a machined piece of hardware that can be assembled into a standard AR type gun that converts the host weapon to fire in full auto mode. The DIAS is considered the machine gun, not the host gun.
See this URL for a more complete explanation:
http://www.quarterbore.com/nfa/dias.html
The Lightning Link is a stamped and folded piece of sheet metal that when cleverly inserted into a standard AR also converts the host weapon into a machine gun.
Here is the URL for an excellent Lightning Link discussion.
http://www.quarterbore.com/nfa/lightninglink.html