I have used many different scopes designed for crossbow over the last 24 years Nikon and Excalibur are very good and hold "0" well. I just bought a Parker Hurricane with a center point Parker scope on it The Hurricane is excellent with a 430 grain arrow I am clocking 370 FPS, the center point scope gave me some trouble. There is a screw underneath the front of the scope that is touchy and if it loosens up you instantly loose "0" and shoot a very large group. This is not to knock the scope but to identify the problem for anyone who has it, THE FIX I figured out is blue lock tight I adjusted the screw until it was tight enough to hold "0" but loose enough to allow the back height adjustment to work then marked so I could find the setting back then took it back apart and lock tight it with blue lock tight. I have been shooting it for a weak since the repair and by trying to shoot more than one arrow have demolished several its a nail driver when it is right.
PS. I usually just view but I thought this info might help someone.