Possible reason is too low feed forces, the gun bounced off or the gun was not running at 90 degrees to the structure.
The shank is not in the centre of the bucked head. Sloping head.
The bucking bar was not at 90 degrees to the structure throughout the process. Probably wrong type of bucking bar.
Concentric rings in head or on structure around head.
Possible reason is too small hole in the die if the rings arise on head or too large if on structure.
Cracks of different forms.
The criteria here is to detect the type, location and degree of cracks. Open types are usually related to improper heat treatment. For the closed types the location and degree are more important. As long as they do not intersect, there is usually no problem. One reason for cracks can also be that the riveting process is too long.
Swelled rivets that open the structure.
This is normally caused by the hole preparation where some chips have moved between the sheets. You must remove the rivet and clean the joint area. In some cases a small gap is accepted but never in tanks or pressure fuselages.
Open head or gapped head.
Possible reason is that the feed force was not applied correctly, with too high feed forces from the bucking bar side or too low from the riveting hammer side. They are acceptable in most cases but not in integral tank section or in pressure fuselages.
Open head on flush type rivets.
This can depend on faulty countersinking. It could be adjusted by using next size of rivet (if possible) and repeating the countersinking. If the head is up too far some kind of rivet shaving is necessary.
Removing the rivet
When you identify one of the faults described above, for most of the situations it is possible to remove the rivet concerned without any costly results. The common procedure to remove the rivet is:
Drilling to remove a solid rivet must be done from the head side. Select a drill one size smaller than the rivet. Punch a small mark in the center of the head. Be sure that the drill is firmly in line with the rivet. Start drilling at low speed by gently pressing the trigger. This makes it much easier to control the drill and the risk of slipping is minimized.
Drill all the way through the head into the shank so a minimum of material is left between the shaft and the head.
Use a pin punch that fits into the hole and snap it off from the shank. A small chisel can also be used but great care must be taken to not damage the skin.
To remove the rivet use a pin punch with a light hammer and knock a few times. Collect the rivet in some kind of cup. If you follow carefully this procedure, the hole should not be damaged, and a new rivet can be used in the hole.