try and looking up a cz52 extractor on gunbroker they are not high $$ and common right now. muttman
I have started a bolt exactally like this for a suomi/thompson type build. So far I've drilled out the bolt (a gift from MTDEW)for a firing pin and cut the bolt down. I'm still debating if mild steel is going to wear well enough or should I use 4130 for the square bolt body. I suspect the mild steel will be fine since it will have a large bearing surface. I thought I may thread the bolt stub into the bolt body because I've set the back of the bolt stub as the hammer stop. I may be making more work for myself with trying to thread the bolt stub and very well may just end up using a couple 1/4" pins to keep it form being hammered loose. Here's a pic of it so far. For an extractor if you're going to make your own bolt, you might look through sarco and numrich to see if you can find a cheap pistol bar type extractor, pin and spring out of a gun like a S&W, beretta, or even a sten like you mentioned earlier. You might also want to look at the sgn-22 article where it shows how to make this style of extractor, although if you could get one for $10 I don't think I'd bother.
In the bolt stub above the hammer will be hitting the back of the stub so the only real wear on the mild steel parts will be from pushing the hammer down and smacking the bolt stop. If my bolt were a different design the front of the bolt could be the part that stops the bolts forward movement, but there's not much metal at the front of a suomi bolt compared to say a sten bolt. I might just go ahead and use some mild steel (a section of a 2" hitch) as a test piece and see how it goes. If it holds up, great! If not then I don't have anything invested except some time and an end mill or two as I got the hitch for free and I can remove the bolt stub from the mild steel body and reuse it.
As far as if a non-inertia firing pin would be legal or not I think the only people that could say for sure are the bafte boys. One of the ways they have ruled an ar15 with some m16 parts(or worn or broken ar15 parts) in it as a machine gun is if it has hammer follow (missing/broken disconnector or use of m16 trigger disconnector and selector for examples) and use 223 rounds with soft pistol primers the gun can be made to shoot auto. Even though doing so is very unsafe. I don't know what firing pin the 9mm version of the ar15 uses, but it has a different bolt and carrier so perhaps they have addressed the weight of the regular ar15 firing being able to set off 9mm rounds. In the sgn-9 articles the author Matthews stressed not to leave out a firing pin return spring as it would result in a possible run away full auto.
I have a pretty heavy firing pin spring, so I don't know if I'll have to go with the inertia type one, I'll just see what it does to a primed empty case if I take out the disconnector and drop the bolt on it. I can't imagine it'd fire the primer, but it might, so thanks for bringing that to my attention!
Take a look at a pps43 bolt. I think you could cut that bolt face shape out of square stock pretty easy. For an extractor sarco has a beretta 92 extractor, spring, and pin for $9.50. Beretta M9 / M92 9MM