It's not that hard to finish assembly, but to your intent: it DOES take enough knowledge & effort to acquire & assemble all the parts that anyone intending to use the finished product for criminal purposes will just buy one (black market most likely).

"OMG someone can make an unregistered gun and kill someone with it!" is hyperventilating drivel. Far, far easier to just buy/steal one. Anyone interested in making one won't be interested in throwing their lives away (arrest/incarceration/execution) by abusing it. Anyone who IS willing to throw their lives away by abusing one won't find any advantage/interest in making one from scratch.