A modular weapon system (MWS) is any weapon equipment which has removable core components (or "modules") that can be reconfigured/interchanged to give the weapon different capabilities to adapt to various applications. Modularity can provide several advantages to military organizations, such as the versatility of allowing units to quickly tailor their weapons to best suit the immediate tactical needs, to quickly repair/exchange malfunctioned components, and to reduce overall logistical burdens and costs. However, it also make the legal tracking and technical categorization of a weapon more complicated as it can now be easily converted into a diverse variety of different forms.

For example, the Stoner 63 is a 5.56×45mm NATO-caliber modular small arm system using a variety of modular components, which can be configured as a rifle, a carbine, a top-fed light machine gun, a belt-fed squad automatic weapon, or a vehicle mounted weapon.

A modular firearm can also be useful for hunters, which then easily can change to a different chambering or barrel length when hunting different species.