That Saba sluggie has IC chokes in both barrels, so any genuine solid like a turned brass bullet will likely take the chokes with it on first firing!

They are clearly designed for 'rifled' slugs, the so-called 'rifling' is designed to compress as the projectile passes through the chokes.

The Saba looks like a good idea: my only advice is to stay away from the hollow lead slugs like the 'Foster' and stick to proper solids like the Brenneke or the Terminator.

I'm unsure the Terminator would fly well without spin-stabilisation, but hey! ...only one way to find out!