Question of legality of buying firearms in parts

If you buy a gun 1 part at a time, ie: barrel, reciever, trigger assembly, stock, and magazine all at separate times at which point are you buying a 'firearm'? You could break it down further and get all the individual parts from modders and replacement sources but there is quite a few parts to buy. My question is, is this a surefire way to beat gun bans, registration, and any local or state laws since no 'gun' is ever being purchased, just scrap metal.