I've been looking to build my own rifle, the only problem is I am under the age of 21. I know you have to be 21 to purchase lowers or complete lowers, so I am trying to clarify the laws regarding this. Is it legal for my brother, who is of age, to purchase the components, build the rifle, and sell it to me? Or would that be considered a "straw purchase"? I would greatly appreciate some advice regarding this situation as I don't want to break any laws! Thank you!

IIRC, the BATFE form asks if the purchaser is the "end user" of the firearm/receiver.

It sounds like you would be the end user IMO.

That is all I have to say on the matter.

Straw purchase.

IIRC, You can buy a complete rifle if you are 18 years or older. You cannot buy a stripped lower or pistol if under 21 in Texas. ymmv Yes, what you are describing with your brother buying the parts would be a straw purchase.

That's what I was thinking. Thank you for the help!

Would an 80% lower be crossing the line?

When I was 18 a bought an AR-15. I liked it but I started replaced parts and eventually I had a whole new gun.

Your brother cannot buy it with the intent to sell it you.

He can buy it, or just the stripped lower, and gift it to you and then you can complete it.

If it's an actual gift, yes.  If brother "gifts" it and then OP "gifts" some money to the brother, it's a straw purchase.

21 from a dealer. 18 from a private party. Unless you live in a Nazi state.

BATFE finally read the actual law they are supposed to be enforcing.

It initially declares no one under 21 can purchase a firearm.
It then creates an exception for 'long guns' for those under 21 and over 18 years old.

Since an 'action' (or pistol grip only shotgun) are NOT long guns (designed to be fired from the shoulder) you must be 21 to purchase either one.

They do not meet the 'long gun' exception.

That is why the "other" box is on the 4473.

USPS has their own definitions based on 'concealability [size]' that they follow.

As peons we have to follow ALL the Federal limits AND state laws (and even local laws in many cases).