But while I'm discussing the good and bad points of WH40k armor, it's a shame not to do something similar with Battletech armor. True, Battletech armor is superior to WH40k Imperium armor by a margin of about two typical gross violations of physics, but like WH40k armor, it has strengths and weaknesses.
From the material on hand, WH40k armor seems particularly weak against kinetic penetrators. Battletech armor, on the other hand, behaves spectacularly ridiculously towards kinetic penetrators. Modern tank shells are explicitly useless against Battlemech armor; the Battlemech rendered the modern main battle tank obsolete:
The current rules explicitly deal with "modern" weapons, referring to "tank rifles." On the first page of Google hits for the "tank rifles battletech" search string, I find this thread on a BT forum discussing them. (You'll note that the user "Cray" puts the BT Gauss weapons at 6,000 m/s based on game mechanics, while I've only been suggesting 3,400-4,000 based on fluff, and also that it refers to a SB.com thread.)House Steiner Sourcebook wrote:A piece of steel no thicker than my finger, strengthened by radiation casting techniques and impregnated with a sheet of woven diamond fibers, had stopped cold an armor-piercing shell. That same shell would have gone straight through a third of a meter of normal steel.
Not only does the Mackie test show, in fluff, a "modern" tank seeming useless against this new technobabble armor, but it's also codified in the rules. A regular Tank Rifle of the size used on 20th/21st century tanks seems to do literally 0 damage to targets using "modern" armor; and the kinetic weapons that do threaten it (the Gauss weapons) have truly enormous kinetic yields. We're looking at 125 kg hypersonic projectiles, as opposed to modern 5 kg hypersonic projectiles.
And yet this armor is quite vulnerable to high-mass low-velocity impacts, and broad-area high explosive shocks. Normally, it's the density of kinetic energy or the density of momentum that tells us how much armor you can penetrate; in the case of BT armor, a 50 ton mecha charging at 10 m/s does about the same damage as a 125 kg shell moving 3,400-4,000 m/s. Interestingly, the kinetic energy densities are nowhere near each other. The momentum of these two is about the same - but the impact areas are different.
This armor is "hard" enough at all velocities to be concerned with momentum density alone - and transmits shock so quickly that the area of impact barely matters. The only thing that matters is that the momentum is delivered in a short sharp shock - and the total momentum that does so. This armor is kinetically superconductive, and it flakes off at a fixed rate when it absorbs too much momentum.
This leads to an interesting corollary: Compared to modern standards, WH40k tanks are more resistant to high explosive weapons than kinetic penetrators. Relative to the same standards, BT armor is less resistant to high explosive vs kinetic weapons - because the impact area simply doesn't matter. Within each giant panel of armor, the momentum will be spread over several square meters anyway.
For this reason, we could say it wasn't quite fair for me to talk about kinetic penetrators so much when comparing WH40k and BT; some of the other BT weapons, the high explosive missiles and autocannons, would not be as disproportionately effective as the kinetic weapons that that could go in one side of a Leman Russ and out the other - and on the other side of the coin, the Imperium's high explosive weaponry might not be as completely futile as its rare kinetic penetrators.