A modern single action revolver has to be fully cocked and then the trigger pulled and held before the gun will fire. Likewise a double action revolver cannot fire without the hammer being cocked or the trigger being pulled all the way through the cocking stroke raising the transfer bar before the gun can fire. You can't much safer than that. Safety buttons on the other hand can getpushed to the fire positionunknowingly and then anything that snags the triggercan fire the gun.