Bonker, On patched round balls, the patch, if fitting correctly, prevents air from escaping around it. The air is simply pushed out the nipple opening. They also go down the bore like a nut on a bolt, gripping the rifling which is usually around .010 to .012 deep on round ball only guns. Guns that shoot sabots or lead bullets have shallower and faster twist rifling. Sabots are also a bit looser fit but the base allows them to expand, IE... "upset" and seal the bore at firing. Same principle as a hollow based mini ball.