This has been bugging me for some time, and I've never been able to find the answer in black & white.

What is the chapter and verse in the BATF rule book that says a 80% completed AR-15 lower receiver is not considered a firearm, just a paper weight?

Who decides that a particular casting is 80% finished and not legally a fire arm, or decides it's 81% finished and you need to go to prison for being an unlicensed manufacturer or for transferring guns without an FFL? How does that magicical 80% get determined? Is it just how a particular BATF agent felt when he got out of bed that day?

Does the BATF sign off on a manufacturer's casting as being 80% or less? What's the drill a foundry has to go through to make cast receivers without fearing a 10 year jail sentence for producing an 81% completed receiver?

Do the manufacturers /sellers provide the buyer with some sort of letter authenticating that it's a "non" gun as shipped?

ATFE decides whether a particular casting/forging is 80% or not. Many manufacturers submit a copy of the casting/forging to ATFE for a determination, but it depends on what model is being produced. As far as I know, for AR types, you can have any 1 of 3 areas completed. 1-mag well, 2- buffer tube, 3-fire component well.

This is why you see a variety of 80% lowers, with different areas which are completed or not.

I remember reading a BATF letter from years ago stating that the requirement for the 80% cutoff is based on the amount of time required to allow the frame/lower to fire 1 shot has to be at least 8 hrs. This doesn't mean the frame/lower has to work properly, or function 100%, it just has to fire 1 shot to be considered a completed firearm.

Manufuactures don't provide letters authenticating it as a non-gun. They have no reason to to provide this information to the buyer. As the seller, all they have to make sure of is that they are not selling an 81%+, so that they don't run into violations of Fed. Laws.

Some manufactures have gone the route of producing the lower/frame and sending it to the ATFE's Technical Branch in Wash. for a determination. The lower/frame must be submitted in the exact configuration it is going to be sold as. ATFE will make a determination as to whether it falls under the 80% level, and if it doesn't will tell what has to be done in order to bring it into compliance.

Remember, this is Federal, your State may have other laws.

Good info, neilfj.
Does anybody have a definitive source document for this? I have scoured the BATFE site looking at the regs and laws and I can't find anything other than the stuff that says I can make a firearm for my personal use.

Any help with this is greatly appreciated.

"What is the chapter and verse in the BATF rule book that says a 80% completed AR-15 lower receiver is not considered a firearm, just a paper weight?"

You have not found it because, there is no such thing as the 80% thingy in Federal law, Regulation, or Ruling.