I agree with you, I can’t explain those big holes. I don’t know if all GAU-8 rounds are identical; perhaps there is a mixture of solid DU (which might ignite and burn away metal on the big holes and explosive (which would blow out & up the rear of the car) I’ve never heard that there is a mixture but I’m not an expert.
DU rounds are for tanks. For light armor, they’d most likely use HE.