I 'gotta play devil's advocate here, Nial. Since, there aren't thousands of blowed up doubles, damascus or steel, English, European, or good old Yank, and the ones that do appear ALWAYS show absolute signs of obstruction, what are we to conclude?
I had a MacNaughton 12, built back before accurate records were kept, lengthened in the chambers and chokes let out, followed by standard 850 BAR reprooof in Birmingham. This was the third trip through a proofhouse for the old girl by the way, it carried previous London and Birmingham proof. Since the gun had been built as an 13 gauge, and measured 12 when it went for the third trip, assume a generous hone took place as well, just not by my 'smith.
Dangerous? Hardly. A non event, if truth be known. I used it for years. Didn't pay too much attention to the ammunition used, if further truth be known.
My lone Darne is out of French proof for a bunch of reasons-pimple bulges, chokes let out, backbore as well. Oddly, chamber length was always 70 on this gun. But, the thin spot in these tubes is .060 wall thickness, and per the manufacturers suggestion, I just shoot the thing. They advised plain old American hunting loads, a bit more pressure being a good thing in an overbore gun.
Those guys laughed at me when I suggested reproof.
Hey, if I come across the right proofhouse veteran, wearing 2 1/2 marks, but, exhibiting longer chambers, I'll cry about what a damn shame it is to the dealer, and see how much the price plunges. If I find the price acceptable, I'll cry some more to see if I can get the Larry Brown discount, as well.
And then I'll walk away with it. No dead bird has ever come back to haunt me about chamber length.
Best,
Ted
