Later models use a single trigger that alternately fires both barrels, called a "single selective trigger" or "SST". The SST does not allow firing both barrels at once, since the single trigger must be pulled twice in order to fire both barrels. The change from one barrel to the other may be done by a clockwork type system, where a cam alternates between barrels, or by an inertial system where the recoil of firing the first barrel toggles the trigger to the next barrel. A double-barreled shotgun with an inertial trigger works best with full power shotshells; shooting low recoil shotshells often will not reliably toggle the inertial trigger, causing an apparent failure to fire occasionally when attempting to depress the trigger a second time to fire the second barrel (this also can happen if the first shell fails to fire). Generally there is a method of selecting the order in which the barrels of an SST shotgun fire; commonly this is done through manipulation of the safety, pushing to one side to select top barrel first and the other side to select bottom barrel first. In the event that an inertial trigger does not toggle to the second barrel when firing low recoil shotshells, manually selecting the order to the second barrel will enable the second barrel to fire when the trigger is depressed again.