There is a little more to it then that but YUP!Live and learn, I think what happens with those dealers that have the new FA, is that they are licensed to manufacture them, but are not allowed to sell them unless it is military/LE, but are allowed to rent them.
That's just an opinion.
the MAC is a really good way to get into NFA and the cost is in the very low 3K range. If you really want a m16 you can easily get one in the 10K range. If it wears out then parts can be easily found and replaced. The only thing that could go wrong would be a KaBoom and the thing getting blown up but if that ever happened I'm sure an expensive gun would be the least of your worries.
Here .... http://www.subguns.com/
And
Here.... http://www.sturmgewehr.com/
Are NFA classified listings to drool over