How does CMP deal with out of stock?

Do they count the amount of orders they have and then count the amount of rifles left and then mark them as out of stock? Like if they have 500 left and 500 orders. I ask cause I sent in a order back in November got the email saying they got it a week later but they went out stock Dec 2 I think. I've heard they send a email or call you letting you know the item is out of stock but I'm not sure if they only do that when they get to your order or they do it right as they go out of stock