CraigC
Sixgun Nut
If we can assume similar design and construction, we can safely assume Bullet 2 is going to penetrate better.If sectional density was useful then if I give you the following data you should be able to tell me which pernitrates better.
Bullet 1: SD = .192
Bullet 2: SD = .242
But you can't without me providing significantly more data. Even if I gave you the weight and diameter (the minimum data needed to calculate the SD) of Bullet 1 and 2 you still would not be able to tell me which would pernitrate better.
First and foremost, there is no simple answer and none of it is black & white. There is no single number that tells us the whole story. Unlike kinetic energy numbers, at least SD is useful, for something.
SD is a useful number but it must be kept in proper context. It tells us nothing all by itself. SD is useful in comparing similarly constructed/designed bullets of differing diameter, to each other. Where I use this most is with hardcast lead revolver bullets but it also applies to rifles. For example, I know that an LBT style bullet in the SD range of .25-.27 will tend to penetrate the deepest. That holds true whether it's a .44 bullet or a .500. Less than that and you give up penetration. Any more and you're in the realm of diminishing returns. That means 355-360gr for .44's and .45's, or 430's for .475's. The .500's tend to get by with less. Lighter bullets can equal penetration if they have a smaller meplat, such as comparing LFN's to WFN/WLN's. Here we're assuming that similar bullets will have similar rates of deformation if the impact velocity is comparable. In other words, proper context.
All that goes changes when copper/bronze solids come into the picture. Here a lighter bullet exceeds the penetration possible with a solid lead bullet, because there is no deformation. I had an argument with a misguided soul who thought copper solids rendered SD moot. Wrong. To say that SD doesn't matter is to say that weight/momentum don't matter and we know this is false. Otherwise, we could use heat treated aluminum bullets and get the same result but we know that isn't going to happen. Copper/bronze solids don't change the importance of weight to penetration, it just moves the scale back.
TKO is the same way, context is everything. You don't use it to compare expanding bullets to solids, or high velocity bottleneck cartridges to big bores. You only use it to compare big bore solids to big bore solids.