Pardon Our Dust.

[Mr Buckshot]

Actually, vehicles have their own actor variants so when one AI gets in the aiming and behavior of the vehicle is influenced in a way unrelated of the AI in it.
There's less BRG marines in SPv2, and they fire less frequently. There is no way to make them smarter when firing their brgs.

Good enough for me

Just for the effect, have some Covenant-weapon-carrying marines too, like in the original Halo 1, a few times you'd see one with a PR or a needler :P.