I agree, those rivets are under set. I've never squeezed a rivet, just banged them with a rivet gun. I spent 2 or 3 hours a day, M-F for three months banging rivets, and got pretty good by the end of my first semester of Air Frame and General while getting my A&P.
I've heard, it takes about an airplane worth of riveting to rivet consistently well, and a lot of RV builders end up drilling out all of their first rivets and re-driving them, so they essentially build the plane twice.
I guess the moral of the story is to practice on scrap metal until you can consistently drive good rivets. If I was going to build a plane that requires a lot of rivets, I'd get a recoilless rivet gun.
I used to have a lot of half-moons on my rivets, until the instructor said to keep the head on the rivet for a second or too after I release the trigger, because the gun keeps hitting for a while after you release the trigger. Once I started waiting, the half moons went away.