If it comes down to a registration scheme, homebuilt firearms will follow the exact same rules as any other. One of the biggest benefits that I can see in building your own is the smile it puts on your face every time you shoot it ... in spite of the fact that the POTUS seems to think that none of us make anything, you can sit back after a good shooting string and honestly say to yourself "Damned straight ... I made that!"
Machining is where the "manufacturing" takes place. Once you have finished the machining, it is definitely a firearm. After that (or before that, if for some reason you wanted to), you can have someone anodize or otherwise finish it for you. As a side note, aluminum may be difficult to hard chrome, but the plater will know for sure.