20 thou is for 9" from the breech at the proof house. It's where your hands are.
Your hands are not 9" from the muzzle. I'll spot the easy to dent concern.
I have a 1973 Cogswell and Harrison 20 bore Konor. The gun was proved at .615 in each barrel. The bores are still .615. One barrel is .019 near the muzzle. I don't think the barrels are buggered. That's the way it passed Proof.
I don't worry a minute about shooting it.

My thoughts,

Joe