That little screw between the triggers adjusts set-trigger weight. Usually, it's sealed (varnish, loctite, etc.) when adjusted to prevent it from coming loose. Usually you may free the screw by applying heat from a soldering iron. When adjusted to proper weight, I'd advise to lock the screw with nail varnish, loctite, etc.
It is possible to fire the rifle in unset mode but then the trigger pull is generally horrible.