How did you get a receiver?
It came from an importer who legally imported the receiver as a Repair/Replacement part.

I heard that you could not legally build a gun on a "R/R" receiver, how is that you are building with it?
Actually, the IMPORTER is restricted from building a firearm on R/R receiver. I am NOT the importer.
I AM prohibited from building a Non Sporting Firearm (as banned by Bush, Sr. in 1989) on this receiver.
I am NOT building a Non Sporting Firearm.
Here is a link to a PDF file of the ATFs determination on these receivers:
Here is a link to a PDF file of the ATFs determination on AUG R/R receivers
You will note in the ATF  letter dealing with AUG/ADI R/R receiver the ATF agent states the only real use for
those receivers is to build a Domestic Sporting Firearm. THIS is what I am doing with my SIG SG551-SP R/R receiver.

I heard I could not possess a R/R receiver since it is considered to be a Non Sporting Firearm
in and of itself?
These classifications are determined by a weapon's number (MORE than 10) of imported components and the presence of "evil features".
The receiver is only 1 imported part and there is NO Flash Hider, Pistol Grip, Folding Stock or Bayonet Lug on this receiver.

I thought these receivers were SPECIFICALLY banned from importation.
They were, however ATF HAS allowed these and a few FAL and AUG receivers in for Repair Replacement purposes only.
Assembly restrictions are placed on the IMPORTER, only. Once legally imported and sold into the market, the only
laws governing them (relative to civilians) are the AWB, and the 89 "Bush Ban". I will be adhering to BOTH these laws in my build.
AGAIN.. the completed weapon will NOT be a non-sporting firearm. It WILL be JUST ANOTHER CLONE, like the
G3, FN-FAL, and AK clones currently being built today. IT'll only be a MORE EXPENSIVE clone, though.

Well ... to do this, you need US made parts.. where are you getting them?
This is a project and not all parts are available... yet. THAT is the purpose of my project page; to keep you abreast of parts as they become
available. They ARE coming. I would not have started this expensive endeavor if I was unsure of that. SOME parts I will either
have made myself or will simply be selling them for other manufacturers.

Are ANY parts available yet?
Keep checking the "project page" for the latest news on this subject.

Are there any special TOOLS needed for this build?
YES. You will need a method of safely installing your barrel. THAT is about it. Check my "project page" for the availability of these
components (and instructions) that will allow you to meet that objective.

Is there anything else I need to know before I start a project like yours?
Just keep your eyes on my "project page". This is a rather expensive clone compared to the G3s, FALs, and AKs out there.
It is expensive to make these parts. Due to these factors, there is a "reluctant market" out there. All THIS means: No MASS production of parts.
When available, grab the parts when you see them. I doubt there will be many runs produced.