amazing how uneducated people are on the subject of so called "ghost guns"...
1 - they're not simply "kits" that you buy online that are complete to assemble; they're either incomplete kits that require parts that aren't easy to acquire (sear, trigger group, etc.); or they're partially milled receivers that require professional gunsmithing jigs and tools to complete
2 - while parts kits don't have to be registered (they're not a firearm), a complete functioning firearm does; regardless where it came from or how it was assembled/built. someone with knowledge on firearm workings and access to/experience with lathe, mill, and various other metal working tools could produce their own complete firearm/prototype... but it still has to be registered. Same for suppressors, easy to make... but still have to be registered.
3 - they're not plastic; while many external/structural parts can be replaced with polymer; internal mechanical parts and those that experience a lot of heat and friction cannot (barrel, firing pin, springs, etc.). A completely plastic gun either simply wouldn't work (most likely) or would blow up in your face...
Given the time, expense, and knowledge required for parts kit/prototype firearms, it's pretty idiotic to assume that it's done solely for criminal intent
My uncle runs a gunshop (ffl dealer), and has been gunsmithing for over 40 years. He's had all kinds of customers with parts kits/incompletes go through his shop. Most takes months, in many cases years to search for and purchase all the required parts (blueprints/diagrams for much of the older stuff are impossible to find, or are extremely rare and come at great expense, so reproducing certain parts is out of the question); after the process is all said and done, we're talking thousands of dollars in expenses, even more for an older automatics (stens, grease guns, mp40s, mg42s, etc.)
