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But, my logic was by going 3" and #5 shot might put me closer to a 12 gauge performance.

Many of the 3" shells lack the velocity.....look closely at the velocity numbers.....Estate (if they're still available) actually duplicated the 12 Ga 2 3/4" load of 1 1/4 OZ shot at 1330 FPS.....

That's a helluve 20 Ga shell.....many other 3" 20s do only 1100 FPS!

I have a 20 Ga 2 3/4" load of #7 shot (believe it or not) that's awesome.....I load IMR-800-X in it and get tremendous speeds on a 1 OZ payload.....If I point the gun right the birds drop.