subcalibre
/ˌsʌbˈkalɪbə/adjective
Of a projectile: smaller in calibre than the gun from which it is fired, and typically discharged from a secondary tube set inside the main barrel.
- Subcalibre ammunition is often used in practice fire to prevent wear to the main barrel..
Origin
Mid 19th century; earliest use found in Boston Daily Advertiser. From sub- + calibre.