What I have read about building pistols, is they can be finicky. I've built carbines and rifles without problems. Since I haven't built an AR pistol (complete upper was purchased for my first) from scratch, I'm not familiar. And since the 7.62x39 breed can be cantankerous, I don't want to experiment with it. What length? I'm not sure of that either. 12" and below, I guess, because I don't know what the best length would be? I liked the topic you guys had going here, and jumped in!