You want the no-sabot slugs {AKA Forster} for that barrel. Modified choke is OK for slugs, although rifled slug accuracy will probably*** be better with a no-choke [cylinder-bore] or a small amount of choke..Improved Cylinder as the maximum choke.
For the sabot slugs, you want either a full-rifled barell, or if you have a changeable choke system, [one with different choke tubes] you want a rfiled choke tube.
*** I say probably here, because every shotgun barrel/choke/load combination is a law unto itself. Case in point is my Maverick 88. With the 28" modified choke barell, slug point of impact is just a wild guess. With the 18-1/2" cylinder bored barell, they will stay within 4" at 50 yards with the PMC brand rifled slug.
Suggestion...buy a small box (5 or 10 slugs per box) of as many different brands as you can find locally. When you find one that works really well, check the lot number of that brand, then go back to the shop and buy as many of that same lot as you can find on the shelf.