Assuming all other variables being equal, the mechanism of interaction between an intact shell and the back of the plate near it's ballistic limit remains unaffected by the overall thickness of the plate (at least at normal obliquity). Which is to say, the energy it takes for a shell to move from just cracking the back of the bulge to completely passing through is constant for any thickness under this model.
Mathematically it can be written like this:
where C is a semi-empirical constant indicating energy difference between the two limits.
Therefore we should be seeing the relationship between Navy BL and Army BL as is shown in Chart n.1
As we can see, there is no simple multiplier to convert between these two, as the difference changes from 250fps at 1500fps BL(N) to just half of that for 2875fps BL(N).
On the other hand, as the absolute thickness of the plate grows, it is usual for it to become heat treated to a lower hardness. This has a side effect of making its material more ductile and able to stretch more before breaking, and so the back of the plate can absorb more energy between cracking and letting the whole projectile through, increasing the energy difference between the two limits. But its unlikely that this phenomenon completely negates the reduction in BL difference at different thicknesses/striking velocities.
Therefore that helpful rule used by german engineers to convert between G(d) and G(s) is but an approximation good enough only for a limited set of conditions (german APC projectile attacking german RHA at 30°).
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