At the time of initial proof the R bore was 16.8mm=.661"; L 16.9mm=.665". The gun was reproved after honing with a bore of 17.0=.669".
Possibly Lloyd might recall the current bore, which I believe would show additional honing.
.021" at 14-16" is thin
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit
My regular recreational skeet gun is a 16g with similar wall thickness. I only use RST 3/4 oz. at 4600 psi (and a similar handload) AND the barrel wears a spring steel and leather hand guard which perfectly covers the thin spot.

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