I have read that, according to some, if you have machine gun parts you can
go to prison for trying to build a machine gun. If that's so, how to they
sell machine gun parts kits?

I'm in process of building an AR-15 and am curious, from what I see, the M16
lower receiver is different than an AR-15 lower receiver. M16 full-auto
parts installed in an AR-15 won't make a machine gun because there is no
hole for the auto-sear and the area is milled different, no room for the
auto sear. So, in an AR-15 receiver, all the full auto parts that you could
fit would let it work normally in safe and semi, but Auto would only let the
hammer follow the bolt forward and you would have to pull the "Charging
handle" to re-cock the hammer. It seems everything except the auto-sear
would work in an AR-15 for safe and semi, selecting full auto would only let
the hammer down without firing.

To me, it seems that jailing someone for having some machine gun part, that
work fine in an AR-15's, is kind of like throwing a man in jail for rape
because he has a penis. Or giving everyone a speeding ticket because their
car is capable of speeding. Crazy!

RogerN