welcome to the world of trying to get a rifle length system to run
....
to get the rifle length 7.62x40 to run I had the best luck with 125 flat base bullets (TNT, Pro-Hunter, etc...), more than 25gr of A1680, and a gas port between .104 and .107"....there are a couple other make run tricks like: reduce mag stack pressure (drags on the BCG), play with buffer spring tension and length, and reduce buffer weight.....
the 20" rifle length gas system is now one of my favorites after I got it to run....very smooth and highest velocities I have been able to achieve with the 125 TNT (over 2,600 fps)....building a 20" Noveske 7.62x40 rifle length gas system this weekend....
good luck,
hope this helps,
kurtz