I see from the FTF site you can order
M-16 M4 TRI-BURST 3 Position Safe - Semi - 3 Shot BurstTrigger Group.
http://www.ftfindustries.com/m16fullautopage.htm
Excuse my ignorance, but wouldn't that be considered a machinegun if the parts where installed and therefore would be illegal without a class 3 license?