devon:
The major concern though is finding a situational way around the extra damage resistance... because there are ways that seem binary rather than gradual (ie, suddenly do significantly more damage against such a target because they're designed to, rather than scaling up to battle-field weaponry grade).
Bypassing a catch altogether doesn't solve this unless you involve a work-around representative of the extra damage.
For example, one could just give them situational armor (only applies against weapons of a certain kind or strength- as appropriate to the critter type... for elephants, anything not AP)... but it's semantics whether that's any different than using Toughness + a Catch, since the effect is essentially the same either way.
Like I said- sticking to the RAW, I don't see another way to handle this easily, than Toughness w/ an AP Catch.
At which point, we're back to debating whether or not AP or Massive Damage is a fair catch.
Umbra:
Fair on all points save one: There is a precise, critical reason why ~0.43 ounces of metal will hurt an animal more from one source than another... the delivery of that metal. More powerful guns manage extra penetration by 2 effects: speed, and shape/balance of the round. Faster, smaller but denser bullets will be more likely to breach a target, and from there do damage. When it comes to humans, and even animals, it gets a bit pointless (exactly how dead can something get? 'extra dead?')... but in this case, a poor shot with the right weapon IS more productive than a good shot with something else...
If the right weapon, for example, is throwing a ~0.43 ounce cone of 4000 degree burning magnesium, for example... vs a 9mm... I'd vote for the shaped charge.
And in the case of explosives, there's some mass effect going on- a .50 cal and a grenade might both be Weapon:5... the grenade's edge is in getting a free zone effect.
In the case of a human, it's a moot point- there's only so much surface area the grenade can affect, and you'll only get hit by so much shrapnel... an elephant on the other hand might well get hit with ALL of the shrapnel, meaning that at the very least, it's extra size is no longer an advantage (sure, it means more meat to take the hit, but it means more hits on the meat in equal proportion).
Make sense?