Gardner-Denver PR123 Independent Rotation Heavy Drifter PR123 Drills can be mounted on light Screw Feed Aluminum Guide Shells for underground long-hole drilling, and have an open boss ring to accommodate the feed nut for the screw feed in the aluminum shell. PR123J Drill cylinders include a solid boss suited to mounting the drills on bolt-down slab backs for use on steel channel Chain Feed Masts for air track drilling. PR123 Drills feature independent percussion and rotating actions. The piston hammer of the PR123 Drills and the rotation motor are operated as independent units. Optimum control of drilling can be provided by independently varying the air volume to the rotation and/or the percussion when drilling in extremely hard or soft rock.