As a homebuilder (some AKs, i.e. Yugo M64, Yugo M70AB2, Hungarian AMD 65, Romy G, CETMEs, FALs) I can tell you that most AK kits used to include FA FCGs, and before TAPCO stopped selling to the public you could easily buy the Semi-auto only FCGs and easily build a Semi-auto only AK. Now that TAPCO only sells to retailers (granted other companies still sell retail) and more and more kits do not include barrels because of some ban I've been told, most folks don't use kits anymore.
But lately I've seen Dp28s, Polish RPDs, and 1919 kits out there. With the DP28 so long as you know what you are doing with an Arc welder for 210 volt you can reweld the old receiver and modify it to remove the FA ability. Granted any terrorist with half a brain would just reweld the receiver and clean it up a little to be far along on their way to a FA LMG.
The Chechnyians were building FA MGs for 37mm out of cargo truck axels, steel pipes and studs, with the use of Drill presses, lathes, and homemade gun drills. Not to mention homemade small parts like triggers and such. The Russians kept running into these little homebrews with frightening results as the Russian soldiers got chewed to pieces by them.
It's far easier to build a FA firearm given it's a open bolt as opposed to a closed bolt. Remember lots of folks talk about STENs but a far better example would be the Soviet PPSH41. I was doing one as a project but after I cleaned the parts up and was looking at the 80% receiver, the grief of getting the sheriff's department to sign off and get the ATF forms just didn't measure up so I have spare parts for when I eventually buy one of the finished carbines built as new. Had the same epiphany for my PPSH43 kit and just turned it into a pistol. As I was looking at the necessary steps to create the semi-auto form, it was I realized a lot easier to do it as an open bolt if I wanted to, but I don't.
A blowback or recoil style MG isn't really all that difficult for the skilled and experienced machinest, it gets trickier when you start rifleing the barrels but a good broach cutter carbide bit and some steady hands and patience would allow you to use a lathe to crank out a low quality barrel.