Effective on what, bad guys? I'd imagine a good 25, 30, or maybe 35 yards. After that, with no choke, the pattern spread can get so large that you may miss altogether, even if you have a good hold, such that pellets are on all sides of the target. With an extra-full choke, I'd feel confident to 40 or 45 yards. Triple naught buck and an x-full would be a mean combo for fighting bad men in close-but-not-indoors situations (thick woods, urban combat). For in the house, the cylinder bore is fine - no shots over 10 or 15 yards at absolute most.
Only way to know for sure though of course, is to go to range and pattern your gun at various ranges. Depends also on which buckshot.. 4, 3, 2, 1, 0, 00, or 000.
As far as actual effectiveness once the hit is made? That depends even more highly upon which shot type, BUT I'll say that for 00 and 000, if you just make a hit, there is plenty of velocity to penetrate to the vital areas out to 50 yards, easily. So making the hit is more important than having a higher velocity. Now with a more marginal choice like #4 or #3 buck, penetration COULD potentially be a problem past 20 yards or so - I dunno really. But an 000 lead pellet of .36 diam. going 1,300 at the muzzle will still be going plenty fast at 50 yards to penetrate to heart/lungs of an average man, I do believe. Bottom line.. if you really think a long-ish shot could be necessary to save your bacon (past 25 yards or so), then use 00 or 000. Choke choice is far less important than shot choice, but full or x-full would be preferred.
FWIW, triple naught 2.75s with improved cylinder bore, is what is loaded in my Saiga-12 in the bedroom.