Auto BCG in civilian AR's?

[Patriotme]

I am thinking about buying an upper from BCM and I noticed that they sell Auto BCG's. I assume that they are legal for the general public to use in their M4's but I wanted to ask if there are any restrictions at all on these BCG's.
I once heard that if you have all of the full auto parts and a weapon to put them in then you technically have a machine gun according to the BATF. Is that true or BS?
This auto BCG would be the only such part that I have if I get it?
By the way, what is the advantage of these BCG?
Thanks,

[Palmguy]

They are legal to own and use (possible stupid state-level exceptions notwithstanding).

[motorolahamm]

Nope none at all colt rifles come with auto bcg's just dont try and add a full auto trigger group.

[Patriotme]

I thought they were GTG but I wan't some confirmation. Thanks for the info.