I've had a bad X barrel which FX tested and replaced. It was a Pellet liner when they first came out. Try shooting pellets out of your slug liner. They are supposed to shoot very well out of a slug liner. If they shoot crappy, then you might have yourself a defective Slug Liner and should contact FX
Also, if both shoot bad, try shooting without the moderator on. I found on another Impact this was giving me terrible accuracy. Shot bad with the modulator ON and dead on with it OFF. Got a new Modulator from FX .
Try process of elimination before getting up with FX. It'll save you a headache.