The Nylon 66 really was an innovative gun for its time, the entire receiver was made from #66 Nylon, also known as "Delrin" This was LONG before they ever came out with the Glock.
If I had a dime for every round of 22 I shot in that gun, and my little Browning BAR .22, I'd be retired on an island somewhere!
Mine saw enough rounds over the years that some parts have worn. It still shoots, but it tends to slam-fire when loaded, and that is dangerous. I've taken it apart and cleaned it, hoping that would fix the problem, but obviously, it's something besides dirt, like worn hammer/sear engagement surfaces.