Technique, just a few specialty tools. Spot welder, hydraulic press, bending jig, bolt cutter rivet squeezer...
Okay, next start to finish build, I'll do pics and a write up. You guys know there is info all over the net for AK builds?? I do build them a little differently than most people in that I weld the front trunion. Purist hate that, and insist that rivets are better. Welding is just easier for me to do rather than risk damaging a barrel assembly.