I think the easiest way to help MCH out would be to ensure that they have at least a guaranteed combo that always increases potency like most classes. Then have the ammunition be an aspected type of dmg that is bonus on top of the dmg there. You could choose the aspect of the ammunition to fit the need of the fight. The bonus dmg would have to be a fair bit imo plus have the aspected ammunition provide some utility of some sort based on the aspect.
Just make split shot a 100% chance to increase slug shots dmg which would have a 100% chance to increase clean shots dmg at the very least... The aspected ammunition would give it some great utility that the class is currently missing which is why Bard tends to outshine it a bit.