Minumum 7.62x39 barrel question

[WHSmithIV]

Does anyone know what the minimum barrel length is for 7.62x39 for all the powder to burn before the bullet leaves the barrel?

For .300 Blackout it's 9". I'm in the process of designing a 7.62x39 AR pistol and I don't want a flame thrower. Any help is appreciated. I've done a lot of searching and still don't have definitive data.

-Will-

I do have a stripped lower receiver that I bought and had the transfer done for but I have not bought any more parts for it yet while I try to find out what is the best AR 7.62x39 pistol I can design and build.