Wow that's really cool. Looks like a RPK sight on steroids.
The 9mm bullets I'm selecting for my starting 9x39 loads are the Nosler 250 gr ballistic tips. I'm basing the throat around that bullet. Of course I am attempting to imitate the real thing as closely as possible so if/when real ammo is available it will chamber.Ive though about building one in 9x39 and I even know a guy with a reamer he'll let me use but I just cant seem to find any heavy true 9mm bullets to use or dies to reload with.