Re: Scrap metal?
I've built a car before. Sure a lot of the parts; engine, transmission, uprights, rack, column, brakes, etc. were from the scrap yard, but the chassis and suspension components were home built from steel tube and plate.
The two problems with your title:
Scrap metal - scrap metal is unusable junk. The metal in this case was salvageable or perfectly usable second hand parts.
Built - Nope assembled from parts. To say you "built" a machine to me means you actually fabricated significant parts or at least did some serious work in making those parts fit. Assembly - sticking stuff together that was intended to go together. Building - fabrication or at least serious adaptation.
So "Brit GUN NUT assembled WORKING SNIPER RIFLE at home out of spare parts." FIFY.