This discipline involves the action and interaction of each and every part, both moving and stationary, within the firearm. Most hobbyists are limited to a familiarity with the difference between a bolt action and a lever action. However, in reality, the subject of forearms function is far more involved. It also includes the manufacturing tolerances of each part and how that affects the function. Since function of a firearm is post-manufacture activity, this also involves how a particular firearm functioned, or failed to function, in any given incident.
These skills are particularly important in cases wherein after-market work may have been done on a firearm. This proved of particular value in the Fred Paez case.