ASSUMING the same bullet construction, bullets of similar SD give similar penetration if they impact at similar speeds.
Yes, we can agree to disagree, but that's quite a few assumptions there. Even bullets from the same family can vary significantly in their construction. Jacket thickness, jacket taper, jacket material, lead chemistry, tooling pressure during manufacturing seating the core, work hardening of the jacket and so on.
The example you mention isn't compelling and would need to be expanded since gel is a poor indicator of real world performance. Unfortunately ballistic gelatin is all we have to make us warm and fuzzy about perceived bullet performance, but in the world of bullet and ammunition development it's pretty much despised. It's one of those things that sales and marketing get all excited about because it's something that consumers get all excited about. It's main use at Remington was to tell us if the cup and core would separate upon penetration, to give some idea as to expansion, and to give some indication of mass loss but not much more than that. When you've shot thousands of bullets into gel you start to lose respect for the validity of the test medium. The FBI test is far more extensive and still gives erroneous results.
Most cup and core bullets will lose mass after impact which changes SD. Bullets in the real world exist in an infinite number of states as they start and finally complete expansion. This is complex and if you were to test numerous bullets with similar construction and SDs the results would be all over the map.
The ELD-X bullets you mention, what's the maximum expanded diameter of each (not the final expanded diameter) and what's the duration of that maximum diameter +/- 5%?
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