Most people think that the trajectory of the 12 gauge rifled slug is close to that of a mortar, and since they don't think they could hit anything past 25 or 50 yards (which is probably true if they don't have a set of sights on their shotgun) they zero for slugs at 25 yards. Unfortunately, this short zero severely limits the effectiveness of the slug firing shotgun. Surprisingly, a slug's trajectory is quite flat out to about 125 yards (assuming the proper zeroing range). The biggest limitation of the shotgun slug is that penetration and trajectory drop off drastically beyond 125 yards due to velocity loss, so its maximum effective range is probably about 125 yards. (I still wouldn't want to be hit by a slug at 200 yards though!)
12ga Foster Type Rifled Slug (G1 = .109)
(20" barreled riotgun with ghostring sights)
Range Velocity Zero = 75 Zero = 100
0 1440 -1.0 -1.0
25 1320 0.7 1.4
50 1200 1.1 2.5
75 1120 ± 2.1
100 1050 -2.8 ±
125 1000 -7.6 -4.1
150 950 -14.6 -10.4
200 880 -36 -30
250 820 -69 -62
300 770 -114 -105
A problem with slugs at the longer ranges is their reduced hard target penetration due to velocity drop and generally soft lead construction.
http://www.frfrogspad.com/shotgun.htm