They're already unenforcable -- against criminals, who steal them (both wholesale and retail, sometimes even from police evidence rooms) and illegally import them.
... or from police weapon lockers.
They also make them. See the case recently in Australia of motorcycle gangs making some (very) effective subguns (automatic pistols and the like).
It's pretty trivial to make a firearm capable of being used to perpetuate crimes against people: they just have to be better than not having a firearm, so looks, and impression of effectiveness, are more important than actually being well made firearms. Someone with crude hand tools can make an AK in a day or two, and that's a fairly capable firearm. The net result of regulation and elimination of privately owned firearms is that only criminals and cops will have guns, and military (and militarized police) will be the main ones with firearms suitable for any sort of tyrannical resistance and/or hunting.
There are already efforts well underway to prohibit any firearm with a rifled bore - you know, something that was invented to the point of being effective in the 15th century, and have been in common military as well as domestic use for 200 years. Still, you can't regulate ingenuity.
This is why they're also trying to control/eliminate consumer ammunition production (by making it prohibitively expensive through the banning of things like smelting lead, making it illegal to import lead, and things like that) - at least in the US.