So my players are blessedly ignorant of the mechanics of Dark Heresy, making my first time running a serious campaign a bit easier (I just ad hoc what I don't know). However, something occurred to me while we were playing last week.

I had pitted them against a Killosaurus Rex (the name they chose for the Carnosaur) and they could barely hurt it. TB of 10, Armor of 2, etc. Their weapons did little until a brave feral guardsman stared down the carnosaur while is charged and, at the last moment, threw a belt of grenades down its gaping maw. Clever, characterful roleplaying and good rolling earned him some XP and fate points. Huzzah.

My questions are these:

Are all animal armors considered primitive, even though it doesn't say so in the description?

Are all animal attacks considered primitive, even though it doesn't say say in the description?

When, if ever, would you lower something's TB for purposes of doing damage (I said three grenades blowing up in its stomach negated any damage reduction the creature had).

Thanks for your help!