I'm always curious how much contrary evidence it takes to make a change in the dogma. After four decades in the hobby, I've concluded there is not that much evidence in the world.
I was taught to use 3F in .50 and smaller, 2F in everything larger 40 years ago, and I abided by that rule for years. Then I made the mistake of wondering where that commandment came from, the reason behind it. As far as I have been able to ascertain, it, like hundreds of other 'rules' about BP shooting, has been passed down without question and against all reasonable evidence since the beginning of time. The reason it is said to be the only safe thing to do is..."just because." It must be right, we've been doing it that way for decades. Well, I decided that for me, that is a bunch of nonsense. A gazilliion shots have been fired contrary to the rule in perfect safety and with excellent results. So I shoot what I want, don't care what the dogma says. I'm experienced enough to do it safely, in spite of the predictions of doom from the naysayers.
Your flat assertion that 3F should never be fired in a 20 gauge is a perfect example of how entrenched "knowledge" is passed along to the newbies even though there is no good reason behind it. Of course, everyone says "listen to your gun, it will tell you what it likes". God help you if it says it likes 3F when the dogma says 2F.
The way all the consecrated tenants of black powder shooting, cleaning, lubricating, ete., etc., etc., are required to be taken on faith, against all reason and evidence, would be a perfect basis for a new religion. Doubters beware.
Spence