At what range, and with what ammunition, can you effectively engage targets the size of a grapefruit?  What kind of smoothbore shotgun do you use?  Can you hit this target five times in five seconds or less?  
I'm trying to get a "feel" for the best slugs in use- not public relations garbage.  Just like Sgt. Joe Friday in Dragnet, "Just the facts, Ma'am, just the facts."
Thanks for your input.
Carter, oldpaladin, out...

Old, I could only hit a target that size consistantly with a scoped, fully rifled slug barrel using sabot slugs at 50 yards. I don't think I could get my dick out of my pants in 5 seconds. MIKE.

Slug selection all depends on the shotgun.  Various smooth bore barrels like various brands of slugs.  The same goes for rifled barrels.  With my mossberg 590 (smooth bore), I can usually group around 3 inches at 50 yards, but thats off the bench.  I's say from offhand, my effective range for consistently making headshots is around 25 yards.

The mossberg rifled barrel (with federal sabot slugs) will group onto your grapefruit out to nearly 100 yards, but that's off a bench.  Rifled and scoped barrels, however, are a poor choice on a defensive shotgun.

As for the 5 shots in 5 seconds, that is entirely shooter dependent.  For some people, that's impossible at all ranges.  For others, it would be nearly routine.  The closest comparision I can give is shooting bowling pins, at 25 feet.  Good pin shooters can clear a 5 pin table in 3 seconds or less, but that's with buckshot loads.  For shooters like that, 5 seconds with slugs is "doable".

As for effectiveness (on humans, I presume), I don't think you're going to see much difference from one type to another.  The deer I've shot with various types of slugs all seem to die very quickly, if they are hit right (shoulder-heart-lung area).  The only real difference is the relative accuracy of the slugs, and the slightly longer effective range of the sabot slugs.  Either way, a one ounce, .50 or .729 caliber soft lead bullet is going to let a lot of blood out and a lot of light in.