Looking at the copper solids I wonder if the wading (sabot) is tough enough for the rifling to really grab. It might be just sliding across the rifling. If the rifling in the choke tubes is similar to others its not very deep and its hard for it to gab and stabilize a slug at full velocity. All my attempts at getting slugs to shoot through a rifled choke where dismal. The fact you got the Federal slugs to shoot is encouraging and I would adjust the sights for them.