1. IN A ROCK DRILL INCLUDING A RIFLE BAR, A HAMMER PISTON RECIPROCABLE RELATIVE TO THE RIFLE BAR HAVING AN AXIAL BORE IN ITS HEAD FOR RECEPTION OF THE RIFLE BAR AS THE PISTON RECIPROCATES, AND A RECESS FORMED IN THE PISTON IMMEDIATELY ABOVE AND COAXIAL WITH SAID BORE, THE RECESS HAVING AN INTERNAL WALL SURFACE OF CONED TAPER, THE LARGE DIAMETER OF THE TAPER BEING AT THE TOP END OF THE PISTON, AND THE RECESS HAVING AN ANNULAR SEAT DEFINING THE BOTTOM END THEREOF; A RIFLE NUT IN THE RECESS HAVING AN AXIAL WORKING ENGAGEMENT WITH THE RIFLE BAR, THE RIFLE NUT HAVING AN OUTER BODY SURFACE OF CONED TAPER CORRESPONDING TO THAT OF THE INTERNAL WALL OF THE RECESS, AND HAVING A SHOULDER AT THE BOTTOM END THEREOF COMPLEMENTING THE SEAT OF THE RECESS, AND THE NUT HAVING A PREDETERMINED PRESS FIT IN THE RECESS WHEREIN THE SHOULDER OF THE NUT IS NORMALLY CLEAR OF THE SEAT BELOW; WHEREIN THE SHOULDER AND SEAT RESPECTIVELY OF THE NUT AND RECESS HAVE A CONED SURFACE RELATIVELY STEEPER THAN THE SAID TAPERS OF THE NUT AND RECESS.