With all of the uppers and lowers available out there, I have questions about assembling a complete rifle.

With a complete upper that came off a functioning rifle, combined with a complete lower, such as a RRA, Bushie, etc, and a complete BCG such as the CMT ones, does headspace need to be checked?  

Or, would headspace only need to be checked if a new barrel had to be mounted to the upper?

A buddy is trying to put together another rifle, using an upper he already has (left over), and we were trying to figure out if it was as easy as him ordering the BCG and a complete lower.

Sorry if this is too obvious.  I've never had anything but complete rifles so I certainly don't know the correct answers, and I don't want there to be a safety problem.

Thanks

Just snap the upper and lower together and you're good to go.  If the upper is properly headspaced, putting it on a different lower won't make any difference at all.

A new BCG with a used upper shouldn't be a problem BUT it never hurts to check headspace on an unknown combination. I like to keep all of my uppers and BCG's together. Every upper has a dedicated bolt and carrier. Just my thing.

Lower is used to hold the upper receiver and the trigger group. Lowers have nothing to do with head spacing etc.

Ok, thanks for the replies so far.  I'm clear that the lower is no big deal, I just wasn't sure if a new BCG installed into the complete upper would require headspace being checked.

Thanks much.