Originally Posted by MontanaRifleman
It doesn't work that way. A smaller cal bullet (same weight) in the same case will not make the same velocity as the same weight of a larger cal bullet. It has to do with the with the size/area of the pressure column.
I will add that I also have a 300 RUM and 300 WSM as well as a 7 RM which has more capacity than the 7 WSM. With H4350 I got about 3000 fps with a 180 bullet in my 24" 300 WSM, With IMR 4831, I got about 3000 fps with a 160 bullet in my 24" 7 RM. Same velocity, smaller bullet. Wtih higher performance powders like Retumbo and RL17 I was able to get about 3100 and 3150 with the 160's in the 7. With RL17 in the 300 WSM I was able to get 3200 fps plus with the 180's.
The larger cal allows a greater flow of escaping gasses through the bore which will in turn push a bullet of the same weight faster.