A pull gun (10) is used for pulling a mandrel (12) through an expandable sleeve (124) positioned in a fastener hole (130, 132) having a countersink (140). The mandrel expands the sleeve (124) as it moves therethrough, causing the sleeve to put the metal which surrounds the hole (132, 130) and the countersink in a state of compression. The pull gun has an axially split, expandable countersink prestressing device (16) having a generally central passageway (38) through which the mandrel (12) travels axially as it is being retracted into the pull gun. The prestressing device (16) has conical end surfaces (46) to prestress the countersink as the mandrel is pulled through the hole. The device (16) has an axially split sleeve retainer (76) extending therein so as to retain the sleeve within the fastener hole when the mandrel is being retracted. The sleeve retainer (76) is in contact with the sleeve externally of the gun and has its inner end secured in the gun. The prestressing device (16 ), outwardly of the gun, has a relatively small outer diameter compared with its inner end and with that of the gun, so that the outer ends (46) can be inserted into hole countersinks having limited access. The prestressing device (16), as well as the sleeve retainer (76), is made of a heat-treated ultra high strength alloy steel. Axially directed splits (48A) formed as slots in the device (16) prior to the heat-treating are narrowed so that they form narrow slits (58) at the outer end during the heat treatment. Spring fingers (42) are formed between adjacent slits (58).