Using an AR-15 as an example, this is what the government considers a "gun". This is a stripped lower receiver and is the only part of the entire rifle that is considered a "gun" and that you must submit to a background check to purchase. You can already buy what they call "80% lowers", which are unfinished versions of this piece that require a drill press or other tools to finish the milling process to make it a usable receiver. They are un-serialized (as they are just blocks of metal), there are thousands and thousands of them already sold, and the government doesn't know where any of them are. Some of them are made out of polymer materials (the same types of plastics that Glock handguns are made of), and are even easier to finish milling out. Once the next generation of 3D printing and affordable CNC machines arrives, it's going to create even bigger gun legislation problems.

Seriously, I would highly recommend that you would look into what it takes to build an AR-15. It will be a truly eye-opening experience to see that we're really talking about a few pieces of metal, some tubing, and a few springs.