@Bart B... I wouldn't say "bad" but they rely on "all things being equal" "ideal situation" a perfect world.
Inherent resonant frequency let's say A440Hz for example, is a given per an objects' mass x tension or pressure. I can get A440Hz out of objects of a different mass (barrel length/string length) by building in a different amount of pressure... the greater the mass the more pressure required to make it cycle at the same resonant freq as an object with less mass (some of that pressure is already built in remember). The duration of the affect has a direct effect and thus bearing on that objects' mass (temporary mass) and therefore resonant freq. As you are aware there is an ever changing set of variables and all we can do is try to bring those variables closer together: Bullet size/weight, powder charge/burn rate, neck tension.
In engineering terms I suppose adding buttresses for support also adds more variables.