Something doesn't make sense here. 65 mm = 2.56" and if that's the original chamber length and your gun measures 2.775" to the forcing cones (using an accurate chamber length gauge) I'd say the factory chambers had been lengthened for 2-7/8" shells to include a slight overlap into the forcing cone which was standard practice with paper shells. And based on what that gunsmith told you in the 90's I'd say he was right. The question is, does your 1873 gun presumably with composite barrels and chambers lengthened by ~ 1/4" have sufficient remaining wall thickness in that critical pressure run over the forcing cones and some inches forward into the bores? Also, were the original cones made longer and less abrupt which would further reduce wall thickness? I suggest you have the walls measured by a gunsmith with expertise and equipment to do that.
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