Lightning Link
 

An we still make them and pay the tax stamp or are they considered the machine gun and therefore we need to find transferable ones.

Gotta find a transferrable one. Usually run 8 grand and up. Pretty high for $1.50 worth of metal. Also, gotta make sure you have the correct bolt carrier. They were measured for the Colt SP1 carrier, and a lot of brands of carriers have about 1/8-1/4 inch too much metal, which can cause the hammer to trip too early, possibly resulting in an out of battery firing of the round. Another issue to consider, the BATF process of transferring a machine gun. If you have a LGS that is experienced in transferring NFA items, it can be a smooth process, however, I have read of horror stories of people going back and forth for over a year because they were going it alone with the BATF. Personally, I would just pay the 350 for a slide fire and call it a day before going through all that for something to which I am going to have to replace a peice of every few thousand rounds.

Easier to just get the real m16 than the lightning link.

If you can find one, in my opinion, the best route is to find a registered full auto stripped lower to transfer, that way you can build it from the ground up how you like it. LWRC has a closed/open bolt lower parts kit that goes from a closed bolt for semi to open bolt for auto. If I could afford all that, that would be what I would want for a shtf gun.