A pull gun (10) is used for pulling a mandrel (12, 150, 180) through an expandable sleeve (74, 144, 180) 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.