Noun
The act of stopping up, or closing, an opening.
(firearms) The process of a bullet expanding under pressure to fit the bore of the firearm, or a cartridge case expanding under pressure to seal the chamber.
Source: en.wiktionary.orgA metal case holds an integral primer to initiate the propellant and provides the gas seal to prevent the gases leaking out of the breech; this is called obturation. Source: Internet
With bagged charges, the breech itself provides obturation and holds the primer. Source: Internet