I switched to my other hand pop rivet gun, and it produced better results. I installed the spar and top and bottom ribs, pop riveting the spar to the shear clips. Everything went well, and the results look good.
The next step is to rivet the bottom rib to the right skin and attach strip. Looking at the last two holes, it will be very challenging to get rivets in there. I think I’ll follow the advise of some other builders (and Vans), and pop rivet the back two holes. Once the aircraft is eventually painted, it won’t be noticeable.