Let say the Government instituted a gun ban tomorrow. Legions of jack booted storm troopers broke down door all over the country. Through all the bloodshed (mostly innocent civilians because the storm troopers had body armor) They got 85% of the guns.
Then a bunch of guys living way out west using their $5000 machine shop AND A LOT OF BASALT ROCK (you know that common dark grey rock that is laying around every where in the inter mountain west) started making receivers, barrels , bolts, etc and with in a few months firearms are as plentiful as before but this time there are no serial numbers or identification numbers for tracking.
In case you are wondering what Basalt Rock as to do with it, will look up the chemical properties of basalt. Almost everything you need for making a fire arm, (iron. aluminum, titanium, manganese) all in recoverable quantities. OK you are going to need some nickle (5 Cent pieces anyone) as well as chromium, and molybdenum but that is it.
A bunch of home furnaces fired by homemade charcoal from trees, and home forges to make the billets (ALL OF THIS LEGAL YOU DON'T YOU KNOW). The billets get shipped off to underground machine shops and the firearm business is off an running. driving the government nutso. YOU DON'T THINK SO, WELL GO BACK AND CHECK HOW MUCH BOOZE WAS MADE DURING THE PROHIBITION. Remember the brewing of booze put a lot of smell in the air so all you had to do to find the still was follow your nose. Clandestine machine shops are a lot smaller and can be hidden a lot easier than a booze factory. Also a clandestine machine shop can turn out legal products during the day to keep the gun cops away then turn out gun parts at night.