Lol. You think there isnt one?
Can't say I've ever heard of cops busting gang bangers and discover caches of fully automatic AKs that never had factory receiver stamps and serial numbers. Every drive by and drug bust I've ever heard of involved semi automatic or half ass rigged weapons originating from known manufacturing companies.
Making a gun is surprisingly easy, most of the heavy machining and hard work required would be for rifle barrels. Even hand gun barrels could be made pretty simple with a basic lathe and center drill and possibly a broaching machine. Might even get away with crude rifling with hand tools similar to taping dies. It's not like they had fancy expensive machines in the 1800s to produce rifled barrels.
Point is this shit isn't going away, ever. It's like trying to ban the wheel. It's common knowledge, everybody knows how to make one. Gun barrels might not be that simple, but the rest is pretty close.
Even a fool with no math/physics background could trial and error things like bolt carrier mass and buffer spring strength, gas port distances, pressures, volumes, etc, and things like that after blowing up a few test samples.
AKs are made of stamped sheet metal. You can make an AK receiver with a $79 12 ton Harbor Freight hydraulic press, an anvil, and a 2x4. If you don't have the ability to mill solid steel for hammers and fire control group parts, you can cut multiples out of sheet metal and laminate them together into a solid piece. That's how it was designed to be made. And the professionally built parts themselves have never been targeted for a ban; worse case all the bans in the world on the guns, you can still mail order barrels, uppers, etc all day long. You can order complete AR parts kits minus lower receiver and carve your own lower receiver out of a piece of delrin if you really wanted to. Not like you care about the ATF rules if you are preparing for a mass murder.
Powder, easy to make as well. Basic harmless chemicals used in every industry. Bullets? Melt down lead/copper pipes, weights, fishing lures, whatever in a crucible and cast your own. Brass? Easy, machine some dies for the shape and punch it out on the same hydraulic press with a torch and some brass tubing.
It will never go away. Technology's impact on society can never be undone. It's common knowledge now.