Ok, I was using a different reference source, so this one directly from Barnes. .308 150 gr at 2900 has 62. pd/ft of momentum. A 200 gr .358 at 2750 is 78.5 pd/ft. Either way the 200 gr Whelen is going to penetrate farther than the 30 bore with this weight bullet. As far as sectional density is concerned, the post's question was relevant to it. Comparison of similar sectional density bullets of different diameters will always result in the bigger bore having the better momentum and penetration without question, if the velocity is high. However, when one considers the different sectional densities of bullets with different diameters having the same energy level, the same momentum value, the same bullet weight, the one with the smaller bore will penetrate farther due sectional density alone. That was the point I was attempting to make is that sectional density does matter even with expanding bullets. It matters within the same bore or the same cartridge. A 200 gr .358 at 2800 has momentum of 80. A 250 gr at 2500 is 89. The energy is 3481 ft/pds and 3469 respectively. The energy levels are identical for all practical purposes. I'll bet anyone though that the 250 gr traveling with less velocity and less energy will penetrate farther in the same medium with like type bullets, expanding or solids. This is due to momentum and sectional density alone.