Activ: you can fire slugs through a mod choke, which is what most Remington 870's come equipped with. IC will usually provide better accuracy, but not always. Best to shoot the gun and find out which works better.
On this home defense model, I can't change chokes. But I still should be able to shoot rifled slugs, I think. That is my question. I have the Rem 870 Express with the 2 shot magazine extension.
The 870 HD is cylinder bore, fine for slugs. I've heard the Winchesters may be a bit more accurate, but I've shot mostly cheap Remmy value paks (15 rds.)
Hornady TAP slugs are awesome. Avoid sabotted slugs, they're for rifled barrels ot rifled chokes I've been told.
May want to try Brenneke KO slugs and Federal Tru-Ball slugs. Seem to be most consistent between the various shotguns at my house. Doesn't hurt that they both seem to be pretty cheap when you can find them.