Stan;
I'll give you my thoughts on this, but keep in mind these are Only my thoughts.
In the US 10 ga is the largest size legal for waterfowl. Many who do not extensively hunt waterfowl, but may want to do so on occasion would prefer not to invest in a 10, so are more prone to use a 12.
With this in mind, in my opinion, it makes more sense to stretch these sizes to their limit more so than stretching a smaller gauge. Shot column length in an unprotected bore would be almost identical for a 2 oz 10 gauge, 1 3/4 oz 12 gauge, 1¼ oz 20 gauge & a 1 oz 28 gauge. A ½ oz .410 load would be just a tad longer. Also, keep in mind that for the same column length the smaller bore will have a higher % of its shot in bore contact. I do realize that plastic shot cups has eliminated a lot of the "Bore Scrubbing" of unprotected shot in the bore.
The smaller bore will have more of its shot in direct control of the choke, which I firmly believe results in the smaller gauges having a higher central core density. This can be considered as a plus or minus, depending upon one's outlook. I personally consider it a minus as I desire the most uniform spread possible.
This is why in my opinion the "Long" shells truly make sense only at the top end when one desires to fire the heaviest load possible. I was not a dedicated waterfowler but did do some. On the ducks which I killed, I used a 2 3/4" 12 with for the most part 1¼ oz of #6 or #5. To use 1¼ oz of shot I prefer the gun to weigh in around 7 lbs, this makes more sense to me in a standard 12 than a long 20. I love a 20 @ around 6 lbs with an oz of shot or a 28 with 3/4 oz but have absolutely no desire to Magnamize them.
I believe the 3½" 12 came into use purely as the result of steel shot due to its lower density taking up more space. For those who are required by law to shoot non-tox, I personally feel they will be well advised to forgo longer shells in the smaller gauges & go to a standard length in a larger size.
