There is a high probability it is a 2.5 chamber. The thing is that a modern 2.75” shell fits in a chamber of that size because of the crimp (roll or pie). Shells are measured by the length before loading. That being said, if a 2.75 or 3” plastic shell fits in the 2.5” chamber and is shot the pressure will increase due to the end of the shell causing a constricted forcing cone. Not sure how dangerous that is but to be on the safe side there is nothing wrong with having it reamed to 2.75 for safety. Or if you reload, trim the hull to 2.5” then reload it.
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