Part 1, the easy part
A while back I said I was going to do this, and I did. So let us talk about the process.
JSD Supply sells an 80% SIG P320 receiver, the MUP-1. The first one I made was not cut for a manual safety, so I picked up another one so I could use a thumb safety.
You will need a jig for completing these receiver. JSD sells that as well. You can also pick up these parts from any number of other dealers. As for other tools, you will want a drill press or similar. As well as some files or other deburring tools. Something like a Dremel tool with various bits will be useful for deburring. Near the end you will have to cut a slot, so you will need to use a file or cutting bit in a drill press, or Dremel, etc.
There is a little animated video detailing the process:
The jig and the 80% receiver each come in their own small box.
The jig comes with a set of drills and four screws to hold it closed. These are cheap drill bits, but just good enough to get the job done. I broke two when I made a P320 using a hand drill. I sold off that first jig, so I bought a new one for this frame. When trying this again with drill press they all survived just fine.
Aside from the 4 screws, the jig is 3 pieces, a two piece clam shell and an insert that is also used to bend the frame rails.
When you put your 80% frame into the jig, it can slide slightly forwards or backwards. This is not an issue, just as long as you don’t let is move when you are drilling. When you close the jig, and secure it, the frame won’t move.
First step it to put the frame in the two sides of the jig and screw it together. Use the top piece of the jig, as a wedge to start the bend on the receiver rails.
Placing the jig on a table and giving the jig a few taps with a rubber mallet will get the tabs quickly bent to around a 45 degree angle.
The next step is a little harder. You loosen the screw on the jig a little and insert that top piece. Using a mallet (or press, vise, etc) press this insert down to bend the rails the slide will ride on down until they bent flat with the clam jig. You will find that the the rails will spring back up from the jig.
Personally, why it was advised against, I just put the jig in a vise to quickly bend the frame rails. I tried using a little 1/2 ton press the first time, and I found the vise quicker and easier. But that is unnecessary, it can be done with a mallet and hard surface.
At this point, the easiest step is over. We will have several more steps to do. From trimming the rails to length, drilling a few holes, cutting a slot, and deburring the frame. We will do that next time.