Regularly "Increased magazine size" increases total ammo you have available for any gun but for the Vex Mythoclast the ammo doesn't seem to work. Without the upgrade you get 21 in a clip and 168 shots and with the upgrade you get 56 in a clip but still 168 shots. One other glitch I'm seeing is the Vex Mythoclast seems to be charging in order to shoot sometimes. I don't think this is normal.