I have written to Lija and Shilen and neither was interested. Wilson has introduced the 7.62x40 which is basically a necked up 223 case shooting a 110gr 308 bullet with a true 308 barrel. They have it up to 20" without a threaded muzzle. It has the advantage of working with standard 223 uppers, lowers and magazines. 223 brass is readily available, which is not quite true for 7.62x39.
I did find Kies Firearms today 7.62x39 FAQs
who sell an 18" barrel, but for me the muzzle brake
is totally unnecessary and just makes weapon disassembly harder. I wrote to them and asked them if they could do a custom 20-24" hunting barrel without threaded muzzle and fluting. We shall see what we get back...
Otherwise I may have to sell my carbine and start over with a custom build.