Actually it's more difficult than most think. First pretty much all the weapons that could be relatively easily modified are already banned here in the states. The AR15 for example needs among other things an auto sear. You'll probably want a milling machine and the plans/dimensions before you take on that project. Even then modified firearms need to be tuned to be reliable. An unreliable firearm, even a full auto one, isn't much good and can get you killed in a fire fight. Even criminals seem to know this which may explain why so few even attempt a full auto conversion.
People were building firearms in the 1200's with far less tools available than today so yes, trying to ban firearms completely would be like trying to ban sex.
It just isn't going to work, EVER. In England where there is almost a total ban since the 60's people are building firearms in their basements, one guy has been arrested for converting starter pistols. Prohibition of alcohol didn't work, outlawing drugs isn't working and gun control has been proven not to work.
Besides we here in the states already have 22,000 local state and federal gun laws. If gun laws stopped gun crime wouldn't we be seeing the affect of it by now?
I think most anti gun loon are either willfully ignorant of the facts and reality or just have a bug up their butts about guns.