Tuning Slugs for Tiapan Veteran

Just bought one. Has a long SLUG CZ barrel . Shoots JSB 25 gr redesign jumbo monster PELLETS great. Tried a couple slugs with poor results. My guns have always liked Knockout but this gun absolutely hates them. I couldn’t count on hitting a pie plate at 30 yards shooting them at 930 FPS which is my FX load.


Can some one point me in the right direction. Slug and velocity . A starting point or two.