Effective killing range: 18.5"=25 yards, 20"=30 yards, 24"=35 yards
Why would there be a difference in effective range between barrel lengths? This has been discussed to death on here and barrel length has very little effect on velocity or pattern in a shotgun when you get past 14.5 inches. The only difference in range between an 18.5" barrel and a 24" barrel would be 5.5". It's a commonly believed myth that shorter shotgun barrels equal shorter effective ranges but generally that's not true.