No, that's not at all what I was saying. I'll clarify...
When I made this comment—
—all that it means (in simpler terms) is that if you push a projectile fast enough, even if the projectile is made of a material that has less strength than the target, there is a velocity at which the projectile will pass through the target material. It's residual velocity, or V
r, (which is how much speed it has left after passing through the target) depends upon its impact speed (V
i) and the V
50 (ballistic limit) of the target material and can be estimated through the relatively simple energy relationship of V
r =
√[(V
i)² - (V
50)²]. Obviously, in the case of the video that you linked to, the 90-grain Liberty Civil Defense projectile had enough energy (velocity) to pass through the armor insert and leave a large impression in the clay block behind it.
Computations like these are usually made using the Alekseevskii-Tate hydrodynamic pressure interface model:
½ρP(V-U)² + YP = ½ρTU² + RT
Y
P = projectile yield strength (Hugoniot Elastic Limit)
R
T = target resistive strength
ρ
P = projectile density
ρ
T = target density
The following transcendental equation, which can be solved to determine R
t in relationship to
α to the density, shear (G
T) and bulk (K
T) moduli of the target material is—
[1 + (ρTU² ÷ YT) · √(KT - ρTα²U²)] = YT · [1 + (ρTα²U² ÷ 2GT) · √(KT - ρTU²)].
Assuming a steady-state flow stress field in the target ahead of the projectile and setting 'm' as the slope of the intact yield strength/pressure curve (where m =
¾ for ductile targets) of the target material, solution of the transcendental equation for
α is—
α = √[(2√3GT) ÷ (2σyT + ½mρTU²)]
—where
α must then be utilized in the computation of R
T —
RT = (7 ÷ 3) · LN(α) · σyT
σ
yT = yield strength of the target material.
The Alekseevskii-Tate hydrodynamic pressure interface model is kind of a neat model because it allows us to do really interesting things like figure out how fast a .38 Special 158-grain FMJRN bullet would need to be pushed to defeat a 10mm thick piece of 500BHN armor plate.
Turns out that our .38 Special 158-grain FMJRN would need to strike the armor plate at 3,766 fps (V
AT) to do so, but when it exits the rear of the armor plate at ≈860 fps, almost all of its mass will have been consumed by erosion and it will exit as a small flat disc weighing just 1.3 grains with a 'length' of about 0.005''!