I've been following a story on a 3-D printed gun.  Now for America I'm like "great guns made of plastic".  But I didn't know you could print a 30 round magazine for say a AR-15, and a lower receiver to change a semi automatic AR-15 (meaning the bullets come out as fast as you pull the trigger) to a fully automatic weapon (meaning the bullets fire till you let go of the trigger or you run out of bullets).