I think there is a distinction between "building" a gun on a factory frame or receiver, with purchased parts, and truly "building a gun" like the OP ask about.
My thinking was the OP ask if any of us could "build a gun" from raw materials, to include machineing the frame, slide, receiver, bolt, make the springs, and drill & rifle the barrel, chop down a tree and make the stock, etc..
Taken in that context, there are very few of us who could "Build" a modern gun from scratch, and a few chunks of steel.
I "built" my first 30-06 when I was 18 in 1962 from a DCM Springfield.
I "built" 1911 National Match guns for 5th. Inf AMU while I was in the service.
I still "build" 1911's, and an occasional varmint rifle or AR-15.
But I don't make the parts.
I just fit them to an existing frame or receiver.
There is a huge differance.