There is a lot of misconception about shotgun shells. As has been stated here, the length of the brass has nothing to do with it. Another little known fact is that when comparing "magnum" loads to "high brass", the magnum actually has a lower velocity. I'm talking about lead here not steel. In shotshells magnum loads have a heavier shot payload, not more velocity.
That's absolutely correct. The old lead duck loads of days past were heavier payloads with a slower velocity. My personal opinion is that they were not as effective as the lighter loads, but that's for another discussion. They did however take very very many waterfowl over the years, so one would have a hard time arguing against them.