The Anniston, Alabama Army Arsenal was mentioned, but it should be further added that they are but one depot storing thousands of arms and ammunition (we're talking WWI/WWII/Korea/Vietnam surplus arms.) This is also the home of the Civilian Marksmanship Program's South Store where they sell thousands of rifles (M1 Garands, M1 Carbines, 1903 and 1903A3 Springfields and Model of 1917 Enfields) every year to the civilian market...along with millions of rounds of surplus and new ammunition (they also sell .22 target rifles, and used to sell custom match built Ar-15's.) This is just the program's south store...their north store is located at Camp Perry, Ohio (and they've tons of guns there...picked up a beautiful M1 Carbine from them last summer
) These are what they have in stock...they've been selling rifles for decades so there are tons of rifles already in civilian hands that are chambered for common military calibers.
As far as constructing new ARs...lower receivers can be milled from an aluminum billet if you have modicum skill level. It was mentioned making cast moldings and screwing them together like a clamshell...this has been done by a private company already (and the BATFE shut them down promptly, but that is another story.) If you scan the internet, you will find that lower receivers have been milled from high density plastics, literally made out of plastic cutting boards. The real difficulty will be the upper receiver and barrels, although there are millions in surplus storage that could theoretically be pulled into service. The real problem will probably be transporting pieces from their storage depots to industrial areas. Small parts can be fabricated fairly effectively from other materials.
I like the idea of stamped metal parts...AK receivers can be stamped and bent very easily...weld, screw or rivet. Again, barrels will be the hardest part.
Any cartridge can be loaded with blackpowder versus smokeless...you just have to know the reloading differences and expect your semi-auto and automatics to very quickly become single shot repeaters. Black powder is incredibly caustic and care and maintenance will increase.
Great topic!