This question has always somewhat perplexed me; if the W and Z boson are force carriers, then why arnt cannon balls force carriers too? Ive read before that the line between force carrier and matter is somewhat arbitrary, but what makes the W and Z fall on the force-carrier side? Sure, they transmit forces, but so do cannon balls. The more obvious interpretation seems to me (not knowing much at all about the subject, I should warn) that they are just massive particles, possibly composite ones such as in the Rishon model. Why isnt such an interpretation preferred? Is it a matter of this making things simpler only on a deep mathematical level, or is there some common-sense argument for this too?