I rigged a CZ reduce length O/U 20. Similar gun.
The Stevens has chrome lined barrels as does the CZ, try the factory full , modified and improved modified chokes with 9 TSS , It should throw a great pattern. You may not need an aftermarket choke. I shoot Improved modified and full in mine. I use my bottom barrel as primary ( no. 1 ). Dial it in at about 275 no 9 pellets in the 10" at 40 yards and enjoy.
Wow!! Thank you, that’s great advice. I really appreciate your insight! It would be great if I didn’t need another choke.