Kingfisher
Here Zombie, Zombie, Zombie...
Just because it looked like it was an M16 doesn't mean it was. Assault rifle sales have been regulated by the U.S. government since 1934. It could have been a variant AR-15 which doesn't have selectable fire.
Even then, this was a TV show that had a strong anti-gun theme.
Certain Prop guns are allowed to have select and fully automatic fire, as long as they are only capable of firing blanks, and have a insert into the barrel and the barrel is also made permanently UN-removable. Also certain prop houses are allowed to create weapons as long as they have the Class III permits from the ATF.
I've held them, I've shot them. They completely suck in comparison to the real things and get gummed up damn quick.