Yes, sorry, receiver extension is the proper term but usually you hear it called buffer tube. I did spell it out in an earlier post so abbreviated it later.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek