Howdy
Originally, back in 1873 the rifling groove diameter of a 45 Colt revolver was around .454. That is what .454 bullets were for. After WWII 45 Colt rifling grooves were changed to .451, to match the rifling of 45ACP. Of course we are only talking revolvers because rifles chambered for 45 Colt are a much more recent phenomenon.
I don't currently own a rifle chambered for 45 Colt, but I own plenty of revolvers chambered for the round. I always use lead bullets that are .452 in diameter, .001 over the rifling groove diameter. I have slugged plenty of 45 Colt revolvers, they are very consistent at .451.
If I owned a rifle chambered for 45 Colt, I would slug the barrel. But I am pretty sure the measurement would be .451 and I would use .452 lead bullets.