Eh, it's obviously made with heavy weapons in mind. Take a standard heavy weapon of ammo type X's clip size. Multiply it. Apply to all weapons of that type. Fixed.
Not nitpick on your suggestion, but that sort of blanket fix doesn't solve any of the problems I have with the Backpack Ammo Supply. First, what makes a "standard" heavy weapon? Choosing poorly here will greatly impact every other weapon held to its power level. Second, "all weapons of a type" are scaled very differently from each other. Fully automatic weapons are generally weaker to compensate for their number of hits. On the other extreme, there are single shot weapons that are gamechangers in their own right. Using a fully automatic weapon as a baseline quickly gives single shot weapons ungodly amounts of ammunition. Conversely, a single shot weapon template will barely have any benefit for fully automatic weapons. Third, multiplication is the very issue I have with the backpacks in the first place.
Picking just one flat number to apply to all weapons is going to have weird edge cases. Multiplication makes the edge cases worse.