Not to be picky, but that would be your groove diameter. The bore should be around .442 inch and a groove of .452 inch. The inside diameter of a sized 45 colt brass is only .480 inch, so even a fired case probable wouldn't be large enough to make a slug that would fill the grooves. I would suspect the slug was close to .450 inch to start with. What was the measurement of the slug before pushing it through the barrel?To my surprise the bore is dead on at .450
If you are shooting this in a rifle that load is probably a lot faster than you think. My 8.0 grain Unique load with that bullet averages around 1100 fps out of a 24" Cowboy and a little less out of a 16" Carbine Classic single shot.The RCBS 45-270-SAA is a semi wad cutter on the heavy side, and with 9-9.5 grains of unique it will simply slam and terminally B-slap a pig at nominal pistol ranges.
I've not chronographed the load, but I'd think it was in the 800-900 fps range.
Mine are cast at about 12 BHN, and I've not noticed any performance difference when they're heat treated to 22-26 bhn.
Pretty much echos my results. With 8.2 grains Unique behind the 270 grain bullet I'm getting 1095 fps average in a 20 inch 1894 Colt Cowboy Limited. Out of the 5.5 inch New Vaquero and the same load, it is almost exactly 900 fps. Need to test the Vaquero again to make sure.My 8.0 grain Unique load with that bullet averages around 1100 fps out of a 24" Cowboy and a little less out of a 16" Carbine Classic single shot.