If the goal is to stop someone in 5 seconds, why would you need more rounds for the higher caliber rifle? Seems like 5 rifle rounds and 12-15 pistol rounds would be enough stopping power.
But if your argument is more chances to hit, well then you're just advocating for more rounds going somewhere other than there target, and 30 (high caliber rifle) rounds is certainly too much to fire out your walls willy nilly.