Right I understand that, but it's still a difficult task to build a reliable and functional semi-automatic weapon from scratch. Most gun legislation happens due to mass shootings, and in these mass shootings it's usually a semi-automatic rifle being used, which is usually what is being regulated. A 12 year old kid can order an upper receiver for an ar-15 off the internet or from a store with no restrictions and then buy a 3d printer for a couple hundred bucks, and in the future may have access to it already as some predict they will become a household item, and have a fully functional and reliable AR-15 in a matter of a few days.

At least with homemade weapons there is some skill involved, and to build a reliable piece capable of being useful in a mass shooting would take at least a couple years for some random kid to hone the skill to build it, assuming he didn't have previous knowledge on metal working. If the problem with gun regulation being addressed is the ease of access to the firearms, then legislation for 3d-printing should definitely be considered, at least a background check for whomever purchases the 3d-printer, or changing the definition of what is a gun to include upper receivers as well.