"Different", hm. Just how different are we talking here? If I'm to disregard the description of that Ammunition critical hit altogether, I necessarily end up with the conclusion that the "CASE redirects explosive damage to the rear armor" part of it doesn't apply to the detonation of explosive weapons either -- and that doesn't seem to be listed as its own rule anywhere else...
So, let's posit a tank carrying both a Gauss rifle and some machine guns and equipped with CASE. The Gauss rifle suffers a critical hit. What happens?
Scenario A: The Gauss rifle's explosive discharge touches off the MG ammo as well, and CASE redirects the entire, probably massive lump of damage rearwards; well, this one we seem to have just ruled out.
Scenario B: The Gauss rifle detonation ignores CASE altogether (because it's not the Ammunition critical hit as such) and, since few combat vehicles have enough internal structure anywhere to survive 20 damage, destroys the tank.
Scenario C: The damage from the Gauss rifle detonation is vented rearwards by CASE. Both the MG ammunition and any shots left for the Gauss rifle itself survive untouched.
There might be a scenario D or E that I haven't considered, but I think the three above cover the main conceivable outcomes. Right now I think the intent is C...