Rivnuts come in two varieties - ribbed and plain. Since the plain are the cheapest, they are the ones most often seen on MIL equipment. If you ever use any, there is a fine line between too loose, and too tight. If they are over tightened during installation, they can deflect to the point that the installation tool's mandrel can be hard to remove. The smart way is to set up the tool on a chunk of scrap that's the same thinkness as the installation will see. It's also possible that the rivnuts you ran into may be aluminum, since they are very common in aircraft, and the installer may have borrowed a few from the USAF. Many installers dip the rivnuts into a sealant before they install them, and that also helps prevent them from spinning. Regardless, if you ever install a bolt into a rivnut, lube the threads with anti-sieze, grease, oil, car wax -- whatever you have. Whom ever removes the nut will thank you.
