I must say...AJ52... may have just given some of the best advice yet.

Allowing your barrel to cool is a must. Last year I was experiencing as much as 3 inches of stringing in my three shot groups when the shots were taken in quick succesion. A warm barrel will certainly result in decreased apparent accuracy.

Second, he is absolutely correct about sighting in at 50yds. Why? Simple, slugs have a very low ballistics coeffecient. In laymens terms that means that they are very succeptable to wind drift. In fact using the lightfields as a comparison,(which admittedly are at the low end of the sabot scale),They are 250% more succeptable to wind drift at 100ydsthan they are at 50 yds. In other words if you are experiencing 1in of wind drift at 50 yds, you would actually be experiencing 2.5 in of driftat 100yds. Of course that is assuming an absolutley constant crosswind which never occurs. Therefore for sighting purposes you want to eliminate wind drift from the equation as much as reasonable possible while still testing at a reasonable distance. Thus the reason for testing accuracy of specific slugs at 50yds. (Once a slug is decided upon then you can and should shoot at greater distances to gain experience with wind drift's effects on that particular slug).

Picture:
Here is a prime example of the effects of rapid shooting and a hot barrel on accuracy...notice that the "stringing" effect occurs in an almost perfectly linear pattern. (This group was shot in quick succesion on a day that the range temperatures were well over95 degrees.)