Which Call of Duty game introduced the 'Gunsmith' weapon customization system?

Call of Duty: Modern Warfare (2019) introduced the Gunsmith weapon customization system.

Gunsmith transformed multiplayer loadouts by letting players modify individual weapons with a wide range of attachments. Players could alter features such as optics, barrels, stocks, grips, magazines, and ammunition, creating weapons suited to different ranges and play styles.

Most guns in Modern Warfare offered up to 60 potential attachments, although only five could be equipped at once. That limitation made Gunsmith a strategic system rather than a simple collection menu: adding one attachment could improve accuracy or range while reducing mobility, aiming speed, or stealth.

Earlier Call of Duty games allowed weapon attachments and class customization, but they did not use Gunsmith’s extensive attachment-based framework. Later entries, including Black Ops Cold War and subsequent games, expanded or adapted the idea. “Gunsmith” should also be distinguished from cosmetic weapon-blueprint systems, which apply preset appearances or configurations rather than allowing unrestricted customization.