I started with a milled yugo M70 frontend from APEX gunparts, after shipping it ended up at $100. I made the rear repair section. I made the sides out of 1/4" plate and the rear trunnion out of some 7/8" plate. I milled the rails in it and welded in the ejector. I welded the side plates to the rear trunnion, and then welded the bottom to the rear "half" After this I drilled the holes for the hammer and sear. I made the receiver too short, so I had to heavily modify the hammer and disconnector to clear the bolt carrier. I made the disconnector myself to add a US made part.
I used a US made gas piston, receiver, magazine followers, a muzzle nut, and the disconnector I made to keep it at 8 or 9 foreign parts, I dunno the ATF's current position on rewelded milled receivers.
The stock I'm going to use is an airsoft stock. I'll make a new mounting system for it. This might add another US part as it was converted from a non gun part to a gun part in the US, but I'm quite unsure if that is consistent across all the BATF agents, doesn't matter much anyways as with the 2 questionable parts added together it is 10 foreign parts worst case, so I would be just at the 10 allowable foreign parts.
To be legal as far as my age is concerned I had my dad weld the rear end of the receiver to the front end. The rear half I made would only be around 40% IMO, but it would never be able to fire a round, so it wasn't a firearm in the BATF's mind. Once welded to the front the reciever was over 80%, actually at 100% as it would be able to fire a round. Mag wouldn't fit, but it'd fire a round dropped into the chamber. The legalities past 80% are fuzzy at best, so I had my dad mill out the welds that were in the way in the mag well and sand the outside welds flat.
I forgot to take pics of the progress, so you'll have to deal with a picture of the near completed rifle.
Holding it up for that last one was quite tough. Got a 16" barrel so right now it is just a huge pistol, soon the stock will come in the mail and I can throw it on there. Now all that is left is to weld up the pits left from all the older welding, grind it flat, sandblast it, and reblue it. Oh, and clean up the lightening cut that was all welded on on one side when I got the kit...
I made the mag catch and the catch holder thing from milled stock, as I am terrible at stamping sheet metal. My dad figured out the overtravel stop for the mag latch, it is just a setscrew.
To make the hammer clear the bolt carrier because it sits near 3/8" higher than normal I made it sit lower in the sear/trigger by taking off one of the sear "lugs" and filing the back of the other one where it contacts the side of the sear/trigger. I also made it much thinner [from the hammer face to the back of it], and made a new disconnector to hold it much lower than the old one would have.
I was having trouble where the disconnector would let the carrier go back over it but not forward until you released the trigger, so it would go back and stay back until you released the trigger. Kinda cool thing to think about. It might dampen the forward stroke of cycling and batter the rounds less... Anyways, I filed off a lot of the top of the disconnector to fix this and now there would appear to be no trigger slap, and a very nice trigger pull, a bit long, and I can fix that, but really nice and smooth.
ETA: Almost forgot, I used a metric pattern FAL grip for it, I had make an adapter to adapt it to the different screw angle but otherwise it seems to be quite stable and it has MUCH better ergonomics than the standard AK grip.
ETA2: Looking at the pics I stupidly put the trigger 1/4" or so too high. LOL I could have easily avoided all the troubles, meh, the trouble increased my understanding of the trigger geometry so whatever... Maybe the lightened hammer will decrease lock time a bunch.
ETA3: Forgot to specify that I'm using an airsoft gun AKS-74 folding stock, and adapting it to the rifle with a different trunnion on the stock. I'll probably do a spring loaded latch for latching it closed so I'll just have to pull on the stock to pop it open rather than pivoting a latch then pulling it open. Also soon the bolt handle will be swapped over to the left side, and I'll have to figure out a safety, maybe an AR15 type pattern...