You want to use Improved or Open choke and make sure it is a rifled slug.

Also make sure you take the time to sight the gun in with a few different brands of slugs. I had problems with my 870 getting any kind of accuracy until I switched to remington ammo.

I don't know about 100yds with a bird barrel and rifled slug though. I can tell you that I am getting 3" groups at 50yds with mine.

MassBowhunter
"Climb Higher, Look Lower, Fall Asleep Less"