Check out the picture here:
It shows how the BB bounces inside the barrel going from left to right. Again, this is theoretical.
I think an easy enough way to check this would be with a microscope. Check the BB before it's shot and after (catch the BB with a net or something) and if there are tiny scratches on the BB after it was fired, most probably, it left tiny scratches in the barrel. And after 10 thousand shots, it will add up to a more or less ruined barrel.