Some very interesting comments so far but my impression is: that nobody sits down with pen and paper and designs a silencer using scientific guidance/formulae or whatever. Unless I am corrected by an expert in the field I will go away with the thought that a 'design' is tried almost by 'rule of thumb' and the results are noted. Variations are then made e.g to internals or body diameter and/or length to see what effect they have and to see if they make an improvement. Can the effect of these changes be predicted? For the most part I do not think so although the experience of the maker could have an influence. But the scientific reason why a change could be beneficial is not known ... it is all down to experience. This process will be/could be repeated until the maker, maybe an individual or a company decides that what they have ended up with is the best they can do within the parameters they are working with .... price, overall size (maybe) and design.
Apologies if my view offends anyone ... I would welcome being corrected and being told how silencer parameters are used to predict an end result rather than a 'let's try this and see' scenario.
Cheers, Phil