Sorry, I beg to differ, even being an ignorant BP noob, because when I load a ball into a revolver's chamber it shaves a ring of lead off the ball. Maybe it's not exactly "squished" as I said, it's still definitely deformed by the act of loading. I can't speak to rifles from lack of actual experience, but given the force required to ram a patched ball down the bore of my Kentucky pistol, I can't hardly believe it remains a perfect sphere either.
Further, the ball engages the rifling during its passage through the barrel and that deforms it. Any little irregularity on a sphere's surface is going to disrupt the airflow around it. The ball's spin counteracts the disruptive force of the disturbed airflow, of course, but the spin rotational speed diminishes with time in the air.