He used the parts kit because otherwise the resulting gun would be illegal in Massachusetts.
Here’s a parts kit for an egyptian AK clone, its everything you need for an AK minus the barrel and reciever (in game, i suppose you would use a pipe for the barrel and forge the receiver yourself. But what is really here? A spring (already in the game, or we could add a small one for crafting), trigger group, 
, I am not sure what that upper assembly is called, but basically you dont sit down and build an AK, you build AK parts and then you assemble them. When I built my bicycle I started with a steel frame and I added parts, same thing here.
First you would forge or craft a trigger group out of scrap metals and stuff, then the firing assembly, then you would attach a barrel, receiver, and put on a wooden stock and trigger guard. You would need a welder, a forge, anvil, hammer, screwdriver, knife (for carving the stock). It would take probably 2 days to put it all together- We know from real life experience that you can put an AK together with minimal tools, the kind you could probably put together by borrowing from a few friends, but it is not a fast or easy thing. Certainly it is within the realm of a desperate survivor who wants an automatic rifle and has the time and tools for it, even if they have never built a gun before.
