One of the great lies- That you can convert an AR15 to full auto with one drop in part, and you can buy those parts anywhere- no paperwork- for a dollar or two.
Truth- there are several parts that are different between the AR and M16. Matter of fact, ANY gun that is "readily convertible" to full auto fire is already regulated/prohibited by the rules of the BATFE.
Yes, you would need a machine shop to convert. See- the parts that you have to add- there is no place for them to fit inside the receiver- so would have to be milled out.
Can you buy a "registered sear" that will make an AR into a full auto? Yep. The part is treated the same as a machine gun, sells for several thousand $$$, requires payment of $200 tax, fingerprints, approval from your police chief, background check, etc.
They also fail to mention that short of using a registered sear, converting an AR to full auto- if you are not a licensed manufacturer, will get you 10 years, Federal pen, no parole, and a $10,000 fine.