First of all, Buckshot does NOT have more "knockdown power" than a slug - unless the quantity of pellets that actually strike the target approx. equals the mass of a slug. A 1 oz. slug weighs 437.5 grain. 00 buck pellets weigh approx. 50 grains each. So, it can easily be seen that it would require 9 pellets - striking the target - to roughly equal the impact energy of a single 1 oz. slug. This, of course, assumes that the slug and the buckshot are fired at the same velocity. In fact, many slug loads have muzzle velocities HIGHER than (most) buckshot loads. A standard 00 buck, 2.75" shell has about 9 pellets total - so ALL of them must strike the target in this example. This will NOT happen, except at point blank range, perhaps. There is an argument, advanced by some, that buckshot has more impact energy because it does not tend to penetrate as deeply (or exit the animal).... therefore it must lmpart more energy than a slug - which, in some cases, passes through. This is mathematically impossible, for the same reasons as above. If the buckshot load (which actually strikes the target) does not possess as much energy in flight as the slug - then there is NO way that it can impart more energy than the slug when it strikes the target.
Second, as to range..... Foster slugs, through a smoothbore barrel, are generally (reasonably) accurate to about 70 - 75 yds. Much past that distance, the Foster slug has lost so much velocity that it becomes somewhat unstable in flight. Further, Foster slugs retain insufficient energy to insure a clean kill (as a general rule, about 900 ft-lbs minimum), beyond about 100 yds.
The numbers for buckshot are even worse. Even considered collectively, the pellets in a buckshot load lose velocity and energy even faster than a slug. Even under the best conditions, it is seldom possible to get good patterns of buckshot much beyond 45 yds. Beyond 50 yds, buckshot tends to be so inaccurate that, most often, no more than 1/3 of the pellets will strike a 10" dia. target. So, it is hardly likely that buckshot will deliver enough energy and penetration at 75 - 100 yds to be able to do more than wound a deer.
There ARE exceptions to this - some guns will pattern buckshot better than others (just as some guns will shoot slugs better than others). It is HIGHLY UNLIKELY, as I have just explained, though, that buckshot could be as effective as a slug - much beyond 40 yds. So, from 0 to about 40 yds, use whichever you prefer - buckshot or slugs (slugs DO have an advantage, though). Beyond about 40 yds, slugs have a HUGE advantage. The numbers do not lie. The choice is yours.
I am sure that someone on this board, when they read this, will SWEAR that they routinely kill deer at 350 yds,or some such ridiculous range, with a smoothbore gun with Foster slugs.... (or buckshot, for that matter). All I can say is what I've said - the numbers (meaning the rules of PHYSICS) don't lie.