A good reply I recieved of the same question from 86er over on the Lever Guns Forum.
"A lot of shotgun ammo dubbed 2 1/2 inch (which is 65mm) is actually 67mm. This is just about the norm in British ammo as well as Italian ammo. FWIW, a true 2 1/2 chamber can handle the 67mm ammo perfectly well. Your shotgun could very well be 2 1/2" in American terms but is actually chambered 67mm which is totally acceptable and within spec. The only other possible scenario that makes sense is that it was 2 1/2 inch and later the forcing cones were lengthened, allowing the gauge some slight forward movement. If it were originally 2 5/8" it would measure that or a tad more, not less. I am an English trained gunmaker so I am intimately familiar with all side by side guns as far as repairs, restoration and fitting but I am much less familiar with the history and nostalgia of the American brands. I can't rule out some oddity in the Parker specs but I've never seen it on the many Parkers I've worked on and at least to me my analogy(s) makes the most sense."
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