Re: AK-47
First step: buy a parts kit.
The process is complicated to the uninitiated. But once you understand it, it's incredibly easy. It's no wonder these things are churned out in every 3rd world country that wants them.
In a nutshell, a parts kit will consist of everything from the original military (full auto) AK47. It will have been de-milled per NFA standards, with the receiver being torch-cut. Some kits will be missing some of the FA parts, just so you will avoid the temptation to convert it back into a machine gun.
All you really have to do is build a new receiver and reinstall all the parts onto it, while replacing a minimum of 6 parts (from the BATF list) with US-made parts. What you end up with is a legal, home-built, semi-auto AK47. And the cool thing is that it's considered a home-built firearm. It need not have any serial numbers whatsoever, unless you sell it. Since you built it yourself, it will never have appeared in any FFL's records. As far as the government knows, it does not exist.
For paranoid people like me, that is a good thing.
Some people simply buy pre-fabricated commercial (US made) receivers and install their parts on it. This, however, does require an FFL just like a regular firearm. I chose to do the slightly more difficult process of bending my own sheet metal receiver from an 80% "flat". It is not considered a firearm as long as it's less than 80% complete. So I bought a bunch of flats, procured a bending jig, and spent a weekend with a friend bending receivers. After that, it just came down to fitting, trimming, assembling, and finishing them.
Myself and a "co-op" of like-minded individuals got together and built a lot of them. And I mean a lot.
We ended up with professional looking builds that we assembled, sandblasted, parkerized, and Duracoated.
We chose to use plug welds to connect the trunnions and rails to the receivers we fabricated, instead of the traditional method of riveting or screwing them together. This was a more difficult process in some regards, but easier in others. It looks very nice, though, and it stronger than the other methods. But unless you have a knowledgeable welder, riveting or screw-building is the way to go.
The average cost per rifle, the way we did it, was probably along the lines of $250. And that included the parts kit, receiver flats, tooling to bend them, compliance (US made) parts, plus all the materials to blast/parkerize/Duracoat them. But we did more than most home builders do. Most people just buy a kit, buy a receiver, buy the compliance parts, slap it all together, and end up with a functional rifle. They will cost anywhere between $200 and $500, depending on which parts kit you buy and what kind of accessories you want to put on them.
It's a fun process, and very rewarding. Not that there's anything wrong with buying a commercially made Romanian AK (WASR or SAR series). But the idea of home-building an AK47 in your garage that is "off the books" and built to your specifications is a really cool thing. I would love to see more people getting into this.
Unfortunately, I think the days of home-building AK47s cheaply are coming to an end. With the latest "barrel ban" having taken effect on January 1st, parts kits are starting to run out that have barrels included. In the future, home builders will have to spend hundreds of dollars on US made barrels for their parts kits, significantly increasing (i.e. doubling) the cost of a build.