Until someone comes up with a way to test barrel regulation in the store, or at the gun show, we are subject to this possible malady of doubleguns. Happens with O/Us, too. We tested a Beretta O/U once that was way off. Called Beretta and they said if it is within 8" it meets their specs, and would do nothing. That's bad.
The good news is that the vast majority of quality, vintage doubles are well regulated. I've tested lots of them and never found one that wasn't right on. My last I thought wasn't, a Sterly 20, it crossfired with 7/8 oz. loads. I sent it to Dean Harris and he tested it himself, called and told me it was fine. It had been regulated for 1 oz. loads, at about 1175 fps. Both barrels shoot spot on with that load. But, he lengthened the chambers and opened the chokes for me whilst there, so it wasn't a wasted "trip".
SRH
