Well I'm gonna chime in one more time here. Let's just assume we pass this proof law to protect us Dummies, like Me. Also assume I happen to have one of those 20gas with a facory 3" chamber. Now in your own words you say it is "Decidedly Not" good to go with modern 3" magnum loads.
Now assume this gun is submitted for proof & "Passes". And you know what I would guess that its odds of passing are about 99.98% positive. here does this now leave our Dummie.
Not only does he believe all 3" shells are created equal, he now has the proof marks to back him up. And just why if a 20ga wears essentially the same stock & frame bar as does a 12ga which was originally built to handle 1Ľoz of shot is 1Ľoz from a 20ga at a slightly lower velocity going to create such massive destruction to either the frame or the wood??
On a lot of US doubles if you look at the frames you will see the bars are identical on both a normal 12, 16 or 20, the standing breech size is varied to fit the bbl breeches. The pressure of the 20 ga against the breech thus doesn't have as much leverage as does the taller 12 ga to bend or crack the frame. The light weight thin walled 20ga Flues can of course be an exception to this rule. "BUT" what if it should stand that "ONE" proof load, would that OK it for a steady diet of 3" Magnum loads. There is simply a lot to consider here, more than some are considering.
