from my readings, one of the factors in the early twentieth century search for a range shotgun and a load for it (such as the original Super-X) was the development of "progressive" powders - more of a ever increasing push of building velocity rather than an sudden explosion and set back of the charge.
the thought as I understand it (and believe it) was that a gentler start and then building of the load's velocity as it moved down the barrel damaged fewer pellets and resulted in a tighter down range pattern.
if you look at modern power manufacturer's descriptions you see fast burning and slow burning powders - or no reference at all to burn rate and nothing about "progressive"
my guess would be -for a good as any of my guesses are - is that "slow burning" is the modern equivalent of "progressive" in marketing speak
now - the reason for the thought-
this year I will be feeding a battery of duck guns - a Super Fox, a 3 frame Parker and a Smith Longrange - all 12's of course with heavy barrels
I think is would be interesting to try to replicate the performance of the waterfowl loads like the original Super-x as near as possible with modern components - Nice Shot or bismuth-- understanding these are not copper plated lead and I will likely use plastic wads - although that is not a given
i have a summer to experiment at the pattern board -
i do have a 10ga load from Pete's great chart that will pattern some old high antimony lead 4's at 91/92 percent at 40 yards out of my tighter than heck British short ten - with IMR7625 as the propellant - so I may start the quest with that powder and that shot or some old copper 6's and can then limit the non-tox testing some due to the cost of the shot-
and before all the nay-saying - no - i don't think i can shoot like Mr Buck and am not intending to sky bust at 80 yards
just looking to have a little fun and see a nice even tight pattern that will do the job if i do mine - and a clean hit or a clean miss rather than a cripple down in the thick part of the marsh
and I do think it would be interesting to see what the old guns can do with as good of a representation of the loads they were designed around as can be recreated now a days
so- with an1 1/4 oz of shot- what load recipe would you try?