Normally you want they most open choke you can get. Usually I go with an Improved Cylinder or a Modified. Some of my guns with certain loads actually shoot a bit better with the modified choke even though it's a bit tighter. For as cheap as rifled slugs are it's worth a try to see how it performs with both.

DO NOT shoot it without a choke installed though!!!!! The ends of the barrels are pretty thin without a choke installed and you run the risk of messing up your choke threads.

Paul