As title. By using the default semi-auto setting, any pilot seat with both primary and secondary weapons will be unable to fire their primary weapon with LMB. The Cobra pilot and gunner will have their machineguns mapped to both LMB and MMB so long as that option is enabled. Toggling fire mode will allow the pilot to use his rockets again, but this does not work for the gunner, who will still not be able to use the grenade launcher.
