The .500 3" NE and the .500 3 1/4" NE were loaded the same - 80 grains Cordite and a 570 grain bullet - the 3 1/4" was not given a larger charge. The longer case simply reduced pressure and velocity. The 3" was 2150 fps for 16 tons, and the 3 1/4" was 2100 or 2125 fps (I can't remember which) for 15.5 tons. The reduction in pressure isn't worth the effort of finding brass.
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