M16 parts kit question

[AR-15Man]

Can I buy an M16 parts kit for the purposes of building a Retro Semi-auto rifle?

What's the legality here because this is not an area that I'm well versed in. I see these parts kits offered online and it occurs to me that they're a barrel, AR-15 receiver and a semi-auto FCG away from being a really nice retro rifle.

[knockonit]

yes, you are correct, can't buy a auto lower without being the chosen few. std. ar15 lower will work, find a retro lower and you will have a repro retro rifle, i have assembled a few and enjoy remembering the day i melted one
best of luck with build, easy as making a cup of coffee