Maximum effective range for pellets:
#4 and larger - 40 to 50 yards
#6 and smaller - 20 to 30 yards
The smaller the pellets - the shorter the range [regardless of the weight of the payload]
Standard slugs from a smoothbore barrel - approximately 100 yds.
Saboted slugs from a rifled barrel - apporximately 150 yds.
This is just off-hand stuff, others may disagree, but it's pretty close.