Is there a point of diminishing returns in regards to barrel length ?
Probably there is but 30-32" even if it's long to nowadays standards, it used to the be common place. Keep in mind that the longer the barrel, the stiffer it has to be to compensate for flexing. Mine is going to be a straight 1.25" tube.
I'm planning to use slow burning magnum powder to take full advantage of the extra length.
I'll be a long road of experimentations before I can break any records!