OK, so 7.62x39 dies usually come with two expander balls. One for .308" and one for .310". Using the .310" expander works perfectly for .311" cast bullets in the WTF. Then, you'll slightly bell the case mouth, just as in loading pistol, lessening the chance of shaving the sides of your cast bullets. I use this once the next are expanded with the .311"expander to bell the case mouths:
http://www.midwayusa.com/product/140...-expanding-die
The only thing I would add to your procedure is one of these:
http://www.midwayusa.com/product/548...0-aac-blackout
Give it a light crimp, and you're good to go :-)
Kevin