Foster slugs are usually dead soft lead, and will often flatten/expand on impact.
Brennekes are a hard lead alloy, with sharp shoulders, and cut neat round holes for a looong way into whatever they hit.
Shoot rifled slugs (either Foster or Brenneke) out of your smoothbore barrel, and try the barrel you have (with an ImpCyl choke tube) at the range and see how it does, and how you do with it.
Shooting slugs isn't necessarily fun, so you might want to try it standing, using a rest to steady your aim. Sitting at a bench might get you beaten up more than is good for you.
If it works, buy more slugs. If not, you might want to try a barrel with sights, or a rifled barrel with sabots. I've never had any trouble getting Brennekes to shoot well out of any smoothbore barrel I tried them in. I don't use Foster slugs because if I have to go to slugs, I want to put a hole through and through something. Other people disagree sometimes about that.
Try what you have first, before you spend any money on anything besides a few different brands of slugs. If it works, use it, if not then look for a different barrel.
hth,
lpl