What part is registered?

[fwm]

I'm newly interested in the AR world and have many questions, but one I can't seem to find an answer to is what part is considered the firearm and is registered? Is it the lower receiver, or the upper receiver? Thanks in advance

 

[Onagoth]

lower

 

[F22_RaptoR]

yep, its the lower on the AR15.

 

[41 Colt]

Just a stripped lower is the registered firearm, even a complete upper with bolt & carrier is just parts anyone can buy.

 

[Jericho]

Y'all beat me to it!