The 3" 20 gauge "Magnum" saw the light of day in the 1950s, well prior to the lead ban. It was in those early years heavily promoted as a "Duck" gun. It was gaining much popularity "Until"the lead ban. The lead ban virtually killed it as a duck gun as the most readily available Duck Loads were all steel in those days. Even in the three-inch length the 20 simply would not hold enough of the steel pellets n the larger sizes necessary to make it a truly viable duck gun. The lead shot ban played no part in bringing about the 20 gauge Magnum.
20 gauge guns chambered for 3" shells had been available in the early 190s & possibly even back into the late 1800, BUT, these were not "Magnums", at least not in the same sense as the current 20s.
