The rivets Zenair uses on their aircraft are actually a countersunk flush mounted type of pulled rivet. The heads on the rivet gun need to be modified so that the rivet will be domed after being set. Their website has different builders resources that explain how to do this. Basically, I heated up the two different rivet gun heads I'd need (one for A4 and one for A5 rivets) with a propane torch until they glowed red hot. After they cooled down, I used a drill bit that was the exact diameter as the rivet I was matching to the gun's head to hollow them out just enough to dome the rivet when it was set. The rivet gun heads were pretty easy to mill this way after being heated up. It took a bit of trial and error. I'd drill them out, then set a rivet to make sure the depth and diameter was correct. It only took a few tries for each style. Zenair supplies plenty of rivets, so there was no shortage of spares. I finished by taking a drill bit by hand and twisting it in the hole in order to smooth it out.
