Are you able to shoot groups? If not, tear it down, and start from scratch. Make double sure all mounts and screws are solid. Some guys use lock-tite.

Then, assuming your scope is not broken, you should be able to shoot a reasonable group with just about any sabot, but some are sure to be better than others. Try something middle of the road velocity-wise like the Winchester 1 oz sabot in 2 3/4" or 3" that are rated at 1350 and 1400 fps.

Shoot from a solid rest at no more than 50 yds. Once you are hitting the target, shoot 3 shots before you adjust the scope. You are only looking at your group not how close you are to hitting what you're aiming at. If the slugs are grouping well enough for you, then fine-tune your scope adjustment.

I've seen guys drive themselves crazy by trying to sight in when the slugs they were using were not grouping in the first place. They would shoot one shot, adjust, and shoot again. Problem was the slugs were not consistent, so it made it look like the scope was broken.

You may need to try a few brands until you get the accuracy you want.