The C-Frame riveting/dimpling tool sold by Avery Tools comes with 2 arbors, one which is machined on one end to fit into a pneumatic rivet gun and flush on the opposite end.
This arbor can be used in a rivet gun as an extended flush rivet set. Takes two people. One bucking and the other using one hand to hold the set centered over the rivet (hold it very close to the end... it wont hurt a bit), and the other controlling the gun.
If you do not have this tool, you will have to find something similar.