most are .750 on the h-bars.
i can't recall what my gas port was on the del-ton--but i only enlarged it by one step and that solved the problem. unfortunatly i can't recall what size bit i used out of a bit box with 300 bits in it. being that your barrel is a 20" my sized port would likely be wrong as 16" and 20" ain't the same on the port.
the only barrels in 7.62x39 i have seen so far that wern't right on the edge of a too small port has been the m4 profile citadel-- which seems to run out of the box on about anything.