Buster Upgrades - picking up and using another of which you have already used will revert the upgrade back to Mega Buster. This is to add some more strategic value to picking up buster upgrades the player already has farther than just impeding the opponent from getting said support item.
Noise Crush - in order to make it less of a "free Hard Knuckle", make it so that the charge shot splits into a spread of three uncharged projectiles at mid-range. This would also make sense with the weapon's theme, as sound only travels so far before scattering into nothingness.