a pull gun is used for pulling a mandrel through an expandable sleeve positioned in a fastener hole . the mandrel expands the sleeve as it moves therethrough causing the sleeve to put the metal which surrounds the hole in a state of compression . the pull gun has alternative extensions or sleeve retainers having general appearances of collet chucks and they are adapted to be positioned adjacent holes being prestressed . the retainers are of relatively small diameter for operation through a drill fixture and in small spaces on a workpiece .