The rifle bbl was proofed for a smokeless powder load of 1.3 grams (about 20 grains) of powder with the standard jacketed bullet. I don't recall offhand just what powder the GBP was & was actually proofed for use with a steel jacketed bullet (St.m.G). I am not seeing the nitro proof for the shot bbls, but one would expect them to be there in 1913. In any case it would likely have beeen chambered for the 65mmm shell.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra