I was one of those you reference that shot 28 gauge in the 12 gauge class long ago. Back then I was still young and only about 150# and the 12 gauge for 250 or more birds was just too much.
21 yards is the center point of the field. When shooting doubles, the second bird could be past the centerpoint. Slow reaction can result in birds from stations 3 - 5 slipping past the centerpoint. Shootoffs meant shooting doubles at every station. At the center stations 3 - 5, the second bird was a real challenge and often beyond 21 yards.
A piece of shot going 1200 feet per second is exactly the same whether fired from a 12, 20, 28, or 410. The bigger gauges have more pellets, which means statistically more chances. As discussed some in this thread, the shot strings differ front to rear in length, which affects pattern density.
With a correct lead, the denser shot string of the 28 gauge provides better results than the 20 gauge and would break the bird into many pieces. The 20 has a longer string and could give a bit extra margin for a suboptimal lead, but only knock off a chip or two sometimes.
My longest runs without a miss all were shot with a 28 gauge. Back then I was shooting 1500 - 2000 rounds a week. So I had the leads burned into my brain. Misses were the result of a lag in reaction or a loss of focus.
I had the good fortune to shoot at the same club and time as Bob Brister and Jess Briley in the 70’s when they were doing their experiments pulling a trailer with a 20’ long piece of paper to study how patterns really looked on a target in motion. This work is what resulted in Jess’ pioneering work with the chokes and making the .410 produce far reliable patterns.
Bob Brister’s book Shotgunning is a very interesting read, though some of the technical info now is outdated.
When I still shoot a round now and then, a 28 is what goes to the range with me.
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