Using the DSA FAL receiver as a guide, I'm jumping into the deep end and beginning mill work on this receiver casting. Let's hope I don't hit any show stoppers, but even if I do this can be a fun time waster I think this is going to take quite a bit of time to do and I'm going to do this in small bite-sized chunks so as not to get burned out on it. So it will probably be next year before I finish it
Here are my observations to start with:
The rear of my casting is warped to the left, and so to true it up I have to cut enough off that it will make the rear of the receiver too thin. Quite a warp! I'm going to compensate by cutting everything at an angle to try to even out the cutting between the front and the back. That should work but the existing starter holes will be useless as-is. Also the magazine will sit slightly crooked in comparison to the barrel. Should not affect function, it doesn't look like it will hurt anything except cosmetics.
I'm using the top "rail" as the starting point for references. There is enough meat all over that a non-warped receiver would require cuts on all surfaces both inside and out. I should therefore have enough meat left to cut all the rails and everything even after truing up the sides. It is also too long and will require cuts from both the front AND the back. Since this makes everything relative, I really don't have a GOOD starting point other than that top rail. That and the magazine well are the only two constants.
Once I true up the sides I'm going to start on the barrel threads. I looked high and low and taps for the FAL barrel threads - 1" x 16 TPI NS (national special) - are not to be found for less than $65 So I'm going to see if I can get 16 TPI out of my mini lathe, and if so I am going see if I can clamp the receiver on the bed in order to use a boring tool arrangement to cut the threads. That will save me 1/4 of the price of a new receiver IF it doesn't look like it will work then I guess I'll just have to buy a tap and deal with it. Could probably sell it on the FAL Files once I'm done with it I suppose.
I'm scratching my head over how to get this properly assembled. I think the build order is barrel threads, locking shoulder, rails, lower. The big trick looks like it is to get the barrel threads timed correctly for a used barrel to "time" at 12 o'clock top dead center when torqued to 80-100 foot-lbs. Which from what I've read means hand-tight to about 10 o'clock. Since I doubt seriously that I'm clever enough to figure out how to properly start the barrel threads to make this time up, even if I did know where to begin, I believe the safe bet is to just cut the threads and then use the standard kit-build process of file/shim on the barrel end to get proper timing. That's the safe money for my skills One rotation of the barrel is about .055" from my measurements. The available shoulder sized FS from DSArms suggest that there are shoulder sizes for "all" timing needs through one rotation of the barrel. They even have custom ground lengths available, which indicates I should be able to set headspace no matter what.
So assuming I can get the threads cut, I will have a starting point from which to set the headspace.
Then after that, there's the "trivial" matter of getting the locking shoulder hole drilled. Fortunately I have two locking shoulders out of my parts stash, so I am reasoning that cutting on these locking shoulders will be easier than trying to find a really wide replacement shoulder. I'm not sure there is an absolute value for this measurement as the headspace is set based on how close the bolt sits to the barrel, so it stands to reason the different shoulder sizes are for variances in the distance "left over" from the barrel threading installation process. Assuming I will need to over-time the barrel, the locking shoulder will have to be the part that gets ground down to get proper fit. I would rather grind that locking shoulder down than find out I drilled the hole "correctly" only to find it is too far from the barrel.
Among the other areas that will be challenging are the rails. It looks as if I'm going to need to buy several small cutters for the rails. It appears that I will need a carbie woodruff-style cutter to reach into the sides to make these rails. That or make a dedicated tool from a smaller carbide cutter that is ground away leaving only a small cutting section. Either way I broke my last carbide cutter so I need to tool up again
For the lower receiver pivot screw, the hole is too small. So I need to cut that hole larger. Seems it would be easy to do with a dremel grinding stone. However I think the best bet would be to use a rotary table on my mill to make it pretty.
Finally, between my two SA FAL kits, I do not have an ejector block. Also the DSA FAL I have uses a less-desireable one-piece ejector. Apparently every other FAL on the planet uses a removeable ejector, which makes replacing a worn ejector a breeze. So I'm going to need to hunt down a correct ejector block and pick up a few spare ejectors. It looks like it is going to be fun trying to locate retaining pin holes correctly!
I can't for the life of me figure out where the semi-only modifications are made. These are apparently on the ejector block itself? I presume that like the VZ-58 there is a sear notch that need to be welded up for semi-only.
My plans are to use a Para "kit" from either DSA or Falcon Arms. I'm aiming for a Congo Para build (pun intended). Prices are steadily climbing and at a minimum, assuming I get my barrel timed correctly (I am assuming I won't) then I will need to buy the lower assembly package deal which includes all the US parts I will need, and a barrel.