Murphy, I would suspect 2.5" chambers in a 1910 gun. I used the old 12-70 chamber gauge expedient of a 20 ga. hull with crimp cut off stuffed in a 12 ga. hull until 20 base rim sits on the opened crimp of the 12. On this 1920 Parker, there was no interference with this "gauge" seating easily by hand in the extractor rim cut and the gun closing easily. I'll get some guff for this but I think it's close enuf as a "GO-NO-GO" to allow me to shoot sub 8000psi 70mm with confidence that I am not creating wild pressure excursions. The four hulls from the actual shots I got off at skeet Sunday looked fine in the crimp area.
jack
