What sort of effect are you looking for???? Effective range is a pretty ambiguous question and impossible to answer unless you give a few more details about what you want to do...here are a couple details that illustrate this problem...
Max range of the 220gr sierra match king slug fired at an intial velocity of 1050 fps is given by the JBM calculator as over 3000yds with a velocity of 603 fps even at that range. Would that vel be lethal with that size bullet?? I think its very possible. So we could say that the round would be effective at killing something(one) at 3000 yds. However when we look at the other figures....drop for instance, we see that at 3000yd the drop is 24,567" or 2047 feet so your angle of fire is going to be pretty critical and hitting the target at that range nearly impossible without electronic aids. So I'd say this is not an effective range for this round in reality. .....
The ballistic computer is found on the JBM site at
http://www.eskimo.com/~jbm/ballistic...raj_basic.html
To further add to the confusion, I've fired my SIG and the 799 remington at 250yds with the whisper and find that range to be easily reached accurately. I think the round is capable of 300-500yds in the right hands with the right weapon. My opinion of course....
Hope that sort of answers the question.
Frank