That's hot.

I don't have any pictures of how to place them, but with the trunnion installed, I could locate and weld in the bolt guide rails.  They just get spot welded into the receiver.  Here you can see the left side rail.  It's also got the shell ejector on it.

Before we continue assembling the receiver, it first needs to be heat treated.  There are three areas on an AK receiver that need to be hardened.  They are the holes where the trigger and hammer axis pins go, and the ejector tip that kicks out the spent shell.  The AK-Builder receiver flat is made from 4130 chromoly steel, and is oil hardenable.  That means, you heat it up, dunk it in oil, and it comes out hardened.  Me being me, I always have a variety of flammable gasses and used oil laying around.  Propane doesn't quite burn hot enough to get 4130 over it's critical temperature, so MAPP gas needs to be used.  First step, heat the area we want hardened.  The holes are spaced far enough apart that they need to be done one at a time.

Once it's as hot as it's going to get, it gets dunked in the oil:

I didn't really think this one through.  Since I don't drink coffee, I don't have coffee cans laying about, so I used a plastic bucket.  It works fine, except that I forgot that it melts.  The ring right at the top of the oil started melting away...oops...

Hardening this way actually makes the metal too hard, and it becomes brittle.  It needs to be tempered.  So into the oven it goes at 500° for 45 minutes.  This will draw the hardness back to a more reasonable level so it's hard without being brittle.  Here's how it looks now.  Much less pretty than before, but since it'll be painted anyway, the discoloration doesn't matter.