I believe the PRB and open sights are a good match. The sights limit the range to what the PRB is effective. Unless you have eyesight from hell.
I see no need for a scope on a gun that shoots a PRB. If your eyesight is getting poor. (ask me about that) Limit your range even more. Get closer. It doesn't seem to limit the hunters in archery season.
All a scope does on a sidelock is let you shoot ranges you shouldn't be.