5. IN A GUN, THE COMBINATION OF A RECEIVER, A FIRST MEMBER MOUNTED IN SAID RECEIVER FOR PIVOTAL DISPLACEMENT BETWEEN TWO POSITIONS, A SECOND MEMBER DISPOSED FOR SLIDING DISPLACEMENT TO AN ASSEMBLED POSITION IN SAID RECEIVER, AN ACTUATOR-DETENT DEVICE INCLUDING A BODY INSTALLABLE IN A MATING RECESS IN SAID RECEIVER SO AS TO BE RELEASABLY RETAINED THEREIN BY SAID SECOND MEMBER, A PISTON TELESCOPICALLY MOUNTED IN SAID BODY AND A SPRING OPERATIONALLY DISPOSED WITHIN SAID PISTON AND BODY SO THAT OPPOSITE ENDS OF SAID SPRING ARE IN RESPECTIVE ABUTMENT THEREWITH FOR RESILIENTLY EXTENDING SAID PISTON RELATIVE TO SAID BODY, A LOOP FORMED ON THE END OF SAID SPRING HAVING ABUTMENT WITH SAID BODY, A SLOT FORMED IN SAID BODY TO PERMIT THE RESILIENT SLIDING EXTENSION OF SAID LOOP THERETHROUGH FOR CONTACT WITH SAID FIRST MEMBER TO BIAS SAID FIRST MEMBER TO ONE OF THE TWO POSITIONS, AND A HOLE DISPOSED IN SAID SECOND MEMBER FOR MATINGLY RECEIVING THE EXTENDING PORTION OF SAID PISTON WHEN SAID SECOND MEMBER IS IN THE ASSEMBLED POSITION FOR RELEASABLY RETAINING SAID SECOND MEMBER THEREIN.