I've always had the feeling that folks were underestimating just how far one could hope to accurately fire 12 Gauge Slugs when they say that they're only good out to 100 yards.
Granted that said folks are most-likely taking several variables into account, such as that if you're hunting and fail to hit your mark, then you most likely won't get a second chance, especially on a moving target.
Still, this refutes the often overstated assertion that slugs aren't accurate past 100 yards, as well as scopes are pointless on slug guns.