Just for the sake of ballistics involved in shooting a large pellet - in this instance, a 000 Hornady buckshot, ie: .350", having a BC of .049. We'll give that pellet a normal high velocity load at 1,300fps and zero it at 50 yards.
At the muzzle 1,300fps with 240fpe, at 50yards still going 933fps with 124fps, at 100 yards 759 with 82fpe, at 150yards 634fpe with 57fpe, at 200yards 531fps with 40fpe, at 250yards 442fps with 28fps and at 300yards it has a speed of 367feet per second with 19fpe.

The pellet path is most interesting.
50 yards zero - 100yards 9.12" low: 150yards 33.26"low: 200yards 79.18" low: 250yards 156.24" low: 300yards 278.24" low.

150yards almost 3feet low and at 300yards it's 23.2feet low. Effective at what range?

Capable of causing personal damage at 300yards - absolutely, and would be lethal to a duck at 300yards - absolutely - but hit it - not very likely - would have to be one very unlucky duck considering it would have to be hit in a vital organ as well and there are only 9 to 12 of that size in a magnum 12 bore load.

Now as to regular shot? I would estimate that a lead #6 shot would could possibly not even fly 300yards if started at 1,300fps mv.