A surge of illegal homemade machine guns has helped fuel gun violence in the U.S.

There needs to be a clear definition of a "Ghost Gun" though. No serial number? Go back by about a century or so and serial numbers were not commonplace. This doesn't just apply to firearms but anything else like musical instruments. No point in penalizing someone just because the firearm was made before today's standards were around; especially when they didn't do anything wrong at any point. Are they making "firearms?" If it's more refined than a pipe on a block of wood or something like such chances are really good they went through the trouble of getting an FFL rated for manufacturing firearms.