I was thinking a while ago about how the upper and lower handguards are considered one part and have to be all US to be considered a replacement.

Well, this raises some questions.  

If together they are one US or one Foreign part, then what if they are half and half? If they both have to be US-made to be considered a US-made component, then wouldn't it follow that they would both have to be foreign made to be considered a foreign component?  

And if so, wouldn't a half and half be in a sort of legal limbo, or possibly cancel each other out, in which case they would help with your parts count by eliminating a foreign component anyway?.

Notably, I'm not worried about the answer, just providing food for thought.  

Both US made = US part

One US made / one foreign made = foreign made

Don't try to inject logic into this, it won't work. The feds make the rules, logic is not required!

Quoted: you made my head hurt.

Yeah, me too.

If you read most laws, they are devoid of logic and will give you a horrible headache.