From my 18.5 in cyl bore, buckshot is consistently effective (most buckshot on target) to about 18 yds with Low recoil LE Tactical ammo. Standard buckshot, at higher velocity not near that far. Federal's new Tactical 00 buck with FliteControl wad produces 9" patterns at 30 yds! 12 inches at 35.
With standard Tactical Slugs such as Winchester Ranger and Fed LE 127RS, hits are possible at 100 yds and beyond. If you want an accurate slug in a smoothbore, try Federal's new Truball. They cut standard slug group sizes in half in my limited testing--they also come in standard and low recoil versions. I tried the low recoil version at advertised 1300fps. Zeroed for 50 yds, a slug will be about 5" low at 100yds.
If you aren't testing the accuracy of your slugs with some semblance of a bench rest, then you're waisting your time and your slugs. Also, start patterning your buckshot on paper at different ranges, such as 5, 10, 15, and 20 yds and beyond. Nobody claims that buckshot is effective against car bodies. Just BG's at close range.
Check out the phone book--Fed. Tac slug at 25 yds. Rather devestating.
Note Federal's new LE 132 00 buck with FliteControl wad. Distance is in Yards.