A lot of guys love the 66 but my experience was terrible. The barrel is held in place by a shim. There's was no way that the shim would stay tight and the barrel kept moving. Since the rear sight was on the receiver, not the barrel, every time it moved, my POI moved 3 or 4" at 50 yds.
I tried to scope one so I could knock jackrabbits off at longer range. Early 80's, I was about 14, could not hit squat with that stupid gun. I shot my Win 1906 better at 50 with open sights than I could that scoped Nylon 66 at 50. Later, after the internet came out, I read that scope mounting isn't recommended due to the receiver cover shifting, or something along those lines.
Do you know how hard it was to find 1", 3/8 groove scope rings back then? The whole thing was a big disappointment.