They are easy to make, are illegal and they work!
I have read that once you install the lightning link (Takes seconds) you will loose your semi-auto function. You will only have "safe" and full auto. Take out the LL (Takes seconds) and you will have your semi-auto function back.
I have read that 1/16" pop metal works well but a used 1/16" circular saw blade works best!
Last and most important....Just the possession of the two interlocking pieces of metal is a violation of either the '68 or '86 NFA (I forget). The LL does not have to be installed in the weapon.
Last, IMO you do not want nor need a fully auto weapon. Semi-auto will do the job! If you need one pick it up from the Blue Helmeted thug you just sent to his/her glory.