As far as the weapons go, I've always had this impression that the Buster was an energy weapon that actually draws on Mega Man's own power supply. The way I figured it was his internal power supply could recharge quickly enough to allow him to perform most tasks without having to worry, but eventually shooting things would drain him enough that he'd have to rest in order to recharge. Other weapons have their own power supply, but they're demonstrably more limited because of that.