The chances are the money is in the form of a check....one billion dollars, even in hundreds, would be 10 million bills, or 100,000 stacks of bills. Assuming the dimensions of the stack are 6"x2.5"x1.5", so the area of each stack would be 22.5 cubic inches, so the area of the room would have to be 2.25 million cubic inches, which is pretty large.
From that, we can determine that the money would be in check form, and most likely, a check can't stop a bullet. Anyway, still go for it, and if you find that you don't have the gun with ammo, just move erraticly so he can't hit you well.